176 THE 8ALTON SEA. 



loss would be great. Similar structures, however, are offered by the swamp xerophytes of 

 the present time, and the ancient species with a habit and structure of sclerophylly were 

 characterized by reproductive mechanisms which made a supply of moisture a necessity. 



If other desert plants were included in the earlier floras their remains have not yet 

 been found. It is to be conceded, of course, that the conditions in deserts are not favorable 

 to the making or preservation of fossils. The comparative fewness of the individuals, the 

 destructive effects of organisms in brackish water, the fierce grinding action of the torrential 

 stream flow, and the looseness of the material laid down, would all act to destroy plant 

 remains and give a sparse record. The wind-blown material deposited as loess would be 

 equally unfavorable for the preservation of plants. Probably the best set of conditions 

 would be those in which a desert stream-way originating in the highlands would carry 

 material from its middle dry course and bury it in the alluvium, as must be taking place in 

 the Colorado Delta at present, yet nothing of the kind has been examined which would 

 justify the assignment of any great age to the xerophytic habit among plants. 



Not a little interest attaches to the conifers in this connection, as the members of 

 this group probably represent the earUest structures which might be suitable for existence 

 in arid climates. The group as a whole is widely distributed with regard to climate and 

 geological history. Some of the species live in cold, others in warm humid climates, while 

 others not widely dissimilar in structure are found in regions of great aridity. Survival 

 under such circumstances has been attributed by Groom to architectural features.' The 

 size, outer membranes, and longevity of the leaves may show a wide range of variation with 

 consequent modification of use and loss of water. It seems highly probable that the trees 

 of this group were the first plants of size to inhabit arid areas, although the xerophytic 

 habit has not been carried so far as to allow them to endure the conditions of the intensity 

 offered by the Sahara or the Cahuilla. Furthermore, it can not be said that the structural 

 features of the coniferae are the ones which have been developed or modified to meet the 

 conditions of greater aridity by other plants. In other words, the most marked and the 

 most pronounced xerophytic developments have taken place by other methods than those 

 which find expression in the conifers. 



It may make toward a better understanding of this matter if it is recalled that the 

 major movement in phylogeny and habit in plants has been away from types with separated 

 gametophytes in which the sporophyte and the gametophyte have an independent exist- 

 ence, toward forms in which the sporophyte vastly outranks the gametophyte in size and 

 carries the bisexual generation within its tissues, living under more or less aquatic con- 

 ditions, toward preponderantly sporophytic forms, capable of effecting reproduction and 

 of carrying on the reproductive functions under progressively arid conditions. The logical 

 development of such an evolutionary tendency would of course produce the xerophytic 

 types last of all, and while it is not easy to fix upon a time when these appeared, yet the 

 weight of inference hes in favor of the assumption that desert vegetation is composed 

 chiefly of modem vegetative types. 



A large proportion of the species which might be included in the flora of a desert 

 region may be distinctly of a mesophytic habit requiring a large supply of moisture for 

 their development. These forms are either localized around the sources of ground water, 

 or they may be annuals or of short cycle coming to maturity rapidly within seasons of 

 periodic rainfall. As has been pointed out elsewhere, the number of types of this kind 

 is very small in the Salton Sink and in the Libyan Desert where the rainfall is highly un- 

 certain as to its amount and time of occurrence. The features of such plants which seem 

 to have a direct connection with their habitat are those of rapid development and the 

 ability of the ripened seeds to endure long desiccation at high temperatures. 



> Rem&rks on the ecology of the cooifers. Annals of Botany, vol. xxiv, p. 241. 1910. 



