Vegetation Groups of the Laboratory Domain. 291 



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 to 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 plants as 

 Apodanthera undulata, Martynia althcr folia, Tetraclea coulteri, 

 Talinum linear e, Rumex hymenosepalus, and Brodic a capitata 

 begin growth earliei and continue later in the season than per- 

 ennial 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 former areas, in 

 consequence of agricultural operations that have been carried 

 on there for many years. The distribution, frequence, and abund 

 ance 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. For 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, and 32 per cent more are intro- 

 duced species growing in the same area, making a total of 84 per 

 cent of this type of annual plant occurring here. There can be 

 no doubt that these plants which continue 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 and foothills. On the other hand, area III contains 24 

 per cent of the indigenous shrubs, and 6 per cent of the dwarf 

 shrubs, and half -shrubs for the four areas, as against 68 pel cent 

 of the shrubs and 91 per cent of the dwarf shrubs and half -shrubs 

 for areas I and II. 



The plants which are characteristic of alkaline situations 

 are 10 in number, all inhabitants of area III. The most impor- 



