VEGETATION GROUPS OF THE DESERT LABORATORY DOMAIN. 105 
moisture in the soil. As opposed to these, plants like Phacelia crenulata, 
Plagvobothrys pringler, Amsinckia intermedia, and Gulia floccosa are able 
to endure considerable drought by virtue of a better developed root- 
system. They are, nevertheless, species of short duration. As hereto- 
fore noted, growth obtains in these plant-groups during the moister 
portions of the year, after which they disappear entirely from the foot- 
hills and mesa country. Those of the winter annual type, which are 
by far the more numerous, begin to grow in the fall with the advent of 
winter rains and prevailing lower temperatures, such temperature con- 
ditions being essential to their germination,! and continue until well 
in April, after which time they cease to be a factor in the floral covering. 
In contrast with these, the summer annuals begin growth during the 
summer rainy season, when maximum temperatures prevail, and come 
to maturity in the course of six weeks or two months. 
The perennial herbs of these same two areas constitute 22.6 per cent 
of their flora. Since this vegetation form includes 28 per cent of the 
plants of the flood-plain, their presence can not be taken as indicative 
of xerophytic conditions. Many of them are, however, extremely drought- 
resistant. Of the perennial herbs indigenous to areas I, II, and III, 20 
per cent are bulbous, tuberous, or fleshy-rooted species. Among these 
such plantsas A podanthera undulata, Martyniaaltheefolia, Tetraclea coultert, 
Talinum lineare, Rumex hymenosepalus, and Brodiea caprtata begin growth 
earlier and continue later in the season than perennial herbs in general 
without storage organs. Their growth, also, is unchecked with dry spells 
which characterize even our more favorable growing periods. 
The data relating to species growing in the Santa Cruz flood-plain are 
of less value than those of the two former areas, in consequence of agri- 
cultural operations that have been carried on there for many years. The 
distribution, frequence, and abundance of numerous plants have been 
changed; exotic species have found their way in, and not unlikely, 
indigenous plants have suffered eradication. The vegetation forms best 
represented are trees, shrubs, long-lived annuals, and, as already noted, 
perennial herbs. Tor the four areas 11 of the 15 trees occur here and 52 
per cent of the long-lived annual plants for the four areas are indigenous 
to the flood-plain, while 32 per cent more are introduced species growing 
in the same area, making a total of 84 per cent of this type of annual 
plantoccurring here. ‘There can be no doubt that these plants which con- 
tinue their growth during our long season find the deep, alluvial soils of 
_ the flood-plain with their greater water-retaining capacity more conducive 
to their existence than the scant, parched soils of the mesa-like mountain- 
slopes. On the other hand, area III contains 24 per cent of the indig- 
enous shrubs and 6 per cent of the dwarf shrubs and half-shrubs for 


'The writer germinated successfully seeds of winter annual species last summer 
with proper conditions of moisture in a refrigerator; while seeds from the same lots 
with the prevailing summer temperatures remained unchanged. 
