The remains of woody plants which have been described from the Ricardo deposits of Last Chance Gulch consist of well-preserved petrified wood of Pinus, Cupressus, Palmoxylon mohavensis, represented by stem and root fragments, Quercus ricardensis, Robinia alexanderi and an undetermined dicotyledon. Fungi, represented by hyphae and spores in a piece of bark, by hyphae in the wood of Pinus, Cupressus and Robinia, and by fruiting bodies in a palm root have been found. The specimens are associated with mammalian remains which have been referred to the earliest Pliocene. The species represented are all similar to, or possibly identical with those now living in the southern United States, which is consistent with their reference to the Pliocene. The disparities of their range, as compared to related living species, indicates that sufficient time has elapsed to warrant assigning them to this position near the close of the Tertiary. The present habitants of related trees and growth rings in the fossil specimens suggest that the Pliocene assemblage grew in a steep sided valley in a region characterized by low rainfall and temperature approximately that which distinguishes the Upper from the Lower Sonoron zone. At the present time the closest approach to those environmental conditions is found in the San Jacinto Range between San Jacinto Peak and Santa Rosa Mountains, California, where palms are found living in the lower stretches, with pines and live oaks occupying the slopes of the upper portions of the valley. The suggested Pliocene climate differs from the present climate of the Mojave Desert in that the former is characterized by less extreme aridity.

Fossil wood is abundantly represented in the Ricardo Pliocene north of the entrance to Last Chance Gulch in the El Paso Mountains. This locality is approximately northeast of Red Rock Canyon. The occurrence of petrified tree trunks in these beds have been mentioned by Stock and Furlong.

The mammalian fauna which has been described from the Ricardo deposits represents one of the largest and most important assemblages from the Pliocene of western North America. Canid, rhinocerotid, proboscidean and artiodactyl remains have been reported from Last Chance Gulch. Additional forms are known from the deposits in Red Rock Canyon. In Last Chance Gulch, mammalian remains have been reported both above and below the horizon containing fossil tree trunks. Canid and rhinoceritid remains were found approximately 25 feet higher in the section in the immediate vicinity of the locality where the petrified trees occur.

Up to this time, there have been no publications dealing with the Ricardo Flora.

Robinia and Palmoxylon form a large percentage of the specimens in the collections and are the only species which were observed in the field. All of the trunks exposed in the area known as the Saltdale Petrified Forest represent Robinia; approximately two-thirds of these trunks are standing. The largest stump is about four and a half feet in diameter at the base and eight feet tall. Fallen trunks range up to 10 feet in length. Root crowns up to five feet in diameter, single stumps some of which are branched, and twin stumps are represented. Much of the rock exposed is filled with abundant small roots which are undoubtedly related to the trunks. A group of Palmoxylon roots, apparently representing one tree, were observed near some of the Robinia trunks.

Palm roots are represented in the collections by specimens with the cortex preserved as well as by a number of fragments of steles. The tissues preserved are described in the order found from the periphery to the center (length of palm root specimens is anywhere from 26mm to 48mm).

The environmental conditions gained from a study of the Ricardo woods corroborates the evidence derived from an investigation of the fauna and suggests a plains habitat with nearby slopes rising rather abruptly from the stream banks.

Palmoxylon mohavensis (new species). Drabble has published descriptions of a number of modern palm roots, but none of the specimens described by him is identical with that of the fossil species under consideration. The species was compared with a number of roots of the modern Washingtonia. Since the Pliocene palm root differs somewhat from that of Washingtonia, and since it was associated with palm stems that differ markedly from those of Washingtonia, it is referred to Palmoxylon mohavensis, the species of palm represented by wood in the deposits.

Pinus kelloggi (new species). Three of the species of Pinus existing on the American continent today possess wood with thin smooth-walled ray tracheids and thick-walled ray parenchyma cells with small half-bordered pits on their lateral walls. These species are P. cembroides and its varieties P. cembroides ophylla, P. cembroides edulis P. parryana, P. baulfourian and P. austata. Environmental conditions indicated by the other species of the Ricardo assemblage would be favorable for P. cembroides or its varieties, and unfavorable for P. baulfouriana and P. austata. Hence histological data together with ecological data indicate that P. camebroides is the modern equivalent of the species represented. No pines of this type have been previously recorded from the Pliocene of California. Pinus kelloggi is named in honor of Miss Louise Kellogg, who assisted in making some of the early collections from the locality.

Cupressus sp. The various species of Cupressus are localized in their distributions at the present time. C. nevadensis now grows nearer to Last Chance Gulch than any other species. This fact, together with ecological data discussed later, suggests that C. nevadensis may be the species represented. However, it could be C. forbesi or C. arizonica. The physical conditions under which these species grow are quite similar. C. forbesi and C. arizonica occur at greater distances from Last Chance Gulch than does C. nevandensis; however, they are more nearly associated with the modern equivalents of the Ricardo species than is C. nevandensis. Cupressus has not previously been recorded from the Pliocene of California.

Quercus ricardensis. The structures of this wood clearly indicate that is is of the live oak group. Although of finer texture than the wood of Quercus agrifolia, it possesses all of the distinguishing characteristics of this wood Fossil wood of Q. agrifolia from the Rancho La Brea deposits is not essentially different from the Pliocene wood. Several species of Quercus have been described from the Pliocene of California.Of these, Q. lakevillensis is represented by leaves of the agrifolia type, and it is probably that the Ricardo wood might be correctly referred to Q. lakevillensis. Since there are no leaves in the deposits, a more conservative course is followed by assigning a distinct specific name to this wood.

Robinia alexanderi, new species. The wood specimens representing this species are quite variable, but the variations form a species of such a nature as to indicate the presence of but one species. Diversities in the structure of the wood are closely correlated with differences in a width of annual rings. The woods of Robinia pseudoacacia and Toxylon pomiferum are so similar in structure that they have until recently been regarded as indistinguishable in the fossil state; but it is possible, through analysis,to refer the Ricardo wood to the genus Robinia. Leaves or pods of Robinia have been recorded from the eastern United States in beds ranging in age from Cretaceous to Pleistocene. This is the first record of the genus in western America. Robinia alexanderi is named in honor of Miss Annie Alexander, whose early collections in this region have added materially to this paper.

Undetermined Dicotyledon. Of the numerous species of diffuse-porous dicotyledous wood with which this species was compared, it is most closely resembles Xanthorylum, suggesting that it may represent a member of the Rutaceae.

Fungi are abundantly represented by hyphae and spores in a piece of bark, by numerous hyphae in the wood of Pinus, and by fruiting bodies in Palmoxylon. Fungus hyphae also occur sparingly in the tyloses of Robinia, and in one species of Cupresses wood.

More than one species of fungus is undoubtedly represented in the bark. A distinct type of sporangum is abundant in hypertrophied parenchyma cells in the cortex of the palm root. Such fruiting bodies are characteristic of the Chytridiales, of which a number of Synchytriaceae are parasites of angiosperms. It is significant that host plants in wet places are most apt to be attacked by such organisms.

In drawing conclusions from fossil wood as to the environmental conditions of the woody plants represented, the species present and the character of their growth rings must be considered.In the case of species which have existed practically unchanged from past geologic ages until the present time, the conditions of habitat under which the species now exist is a clue to the conditions under which they previously existed. In the case of extinct species their nearest living relatives are the best available index. Since conditions which now favor or check development of a species may be assumed to have had the same effect upon the development in the past, and since such conditions are often reflected by growth rings, they may be an important source of information regarding the environmental conditions of fossils woody plants.

Of the woody species from Last Chance Gulch, one is a monocotyledon and hence without growth rings, two are gymnosperms with well-marked growth rings, and three are dicotyledons with growth rings. The observed variations in width of growth rings of the different species is given in the following chart:

Species	Min. Width	Max. Width	Mean Width
Cupressus	0.500mm	1.54mm	1.05mm
Pinus kelloggi	0.090	2.00	0.49
Quercus r.	0.375	1.20	1.00
Robinia a.	0.450	6.50	2.22
Undet. dicot.	1.000	2.00	1.20

From the above table it may be seen that the greatest width of growth rings is found in Robinia, suggesting that this species made the most rapid growth. The mean width of the annual rings of all species and the minimum of the most rapidly growing species, Robinia, signify that growth in general was rather slow. However, occasional rapid growth is indicated by the maximum width of growth rings found in Robinia. The data in show that most of the specimens of Robinia exhibit only narrow or wide growth rings near the center of the stems, although both types are present in some specimens. Since age of the stem apparently had little effect in determining the width of growth rings within this species, such variations as are observed may be attributed to different environmental conditions. The relative importance of the various ecological factors involved in causing variations in rate of growth can be determined only after consideration of the habitats of the species represented as judged by the habitats of modern related species.

According to Sudworth, P. cembroides, the living species most closely similar to P. kelloggi, ranges from SE Arizona and S New Mexico to S Mexico and Lower California. P. C. edulis distributed from W Texas through New Mexico, western Colorado, eastern SW Utah, N and E Arizona, N Mexico and northern Lower California. P. C. monophylla is found from S. Idaho, eastern Utah and N Arizona through Nevada to the eastern slopes of the southern Sierra and thence into southern California and northern Lower California. Throughout its range, the species including its varieties grows on dry foothills, mesas, mountain ranges, and precipitous canyon sides in poor, shallow, rocky or gravelly silt often in the crevices of rocks.

The presence of a stream in the region characterized by high temperature sand low rainfall would furnish the basis for an explanation of the diversity in width of growth rings observed in Robinia. Under such climatic conditions the rate of growth would be expected to decrease as distance from the stream increased, and differences in growth rate would be noticeable on a slope rising abruptly from the stream bed. The presence of some Robinia specimens with very narrow growth rings and others with wide growth rings suggests that those which made rapid growth grew near a stream bank, while the slow growing trees grew well up on the slope siding rather abruptly from the stream bed.

The mean width of the growth rings of the fossil specimens of Quercus ricardensis, Pinus kelloggi and Cupressus indicate that these species made slow growth. Since the growth rate of Robinia seems to be correlated with differences in water supply available too the different specimens, it is not unlikely that the slow growth of Quercus, Pinus and Cupressus may be largely attributed to their having grown at some distance from the water table. Due to inherited tendencies of the species, growth rates of different species may be quite different under a given set of environmental conditions. This makes it difficult to positively establish the relative positions of the various species, since all but Robinia and Palmoxylon are represented only by small pieces of wood. In view, however, of the low minimum and mean width of the growth rings of Pinus, Cupressus and Quercus, it may be assumed that they grew well above stream level and at a point considerably up the valley from the situation occupied by Robinia and Palmoxylon. This assumption find support in the present distribution of related species in the San Jacincto Range between San Jacinto Peak and Santa Rosa Mountain. Here, Washingtonia is found in the lower stretches of the valleys, with Pinus cembroides monophylla and Quercus agrifolia occupying successively higher positions, for the most part well above stream level and toward the heads of the valleys. Cupressus does not occur at this locality, but is found in eastern San Diego County where it grows under conditions favorable for the growth of Quercus agrifolia. Hence the habitats of the modern equivalents suggest that Pinus kelloggi, Quercus ricardensis and Cupressus sp. grew at a considerably higher elevation than Palmoxylon and Robinia,and on slopes well above the stream.

The relative abundance of specimens of the various species found in the deposits corresponds to that which would be expected under conditions similar to those suggested above as already stated; the numerous stumps at the Saltdale Petrified Forest all represent Robinia, although Palmoxylon roots occur in a position indicating that this species also grew at the locality. The other species are known only from relatively few and comparatively small specimens in the collections. Since the chances for deposition of plant fragments are greatly decreased as their distance from a site of deposition is increased, the relative scarcity of Pinus, Cupressus and Quercus in the deposits may be attributed to their having lived on the slopes well upstream. All available evidence indicates that Palmoxylon and Robinia grew at a comparatively low elevation in close proximity to a stream with the result that wood of these species has been preserved in comparative abundance.

The environmental conditions gained from a study of the Ricardo woods corroborates the evidence derived from an investigation of the fauna and suggests a plains habitat with nearby slopes rising rather abruptly from the stream banks.