39 



The shallower footed forms may indeed be of a more desert 



type. 



3. The region of the bajadas and mesa-like slopes which lies 

 between, supports a flora which is typically xerophytic, for here 

 during the major part of the year the water supply is at a mini- 

 mum. 



There can, in a broad sense, be distinguished three types of 

 desert vegetation. First, the water storage type with its massive 

 parenchymatous tissues and with the practical absence of leaves 

 of which the cactus is the best example: next the type which 

 meets the dry season by dropping its leaves and entering into a 

 resting condition, out of which it may awaken with surprising 

 speed when the rains come. This is well represented by Fou- 

 quiei'ia splendens, sometimes incorrectly called the "cane 

 cactus." The last type is that which has leathery leaves which 

 it maintains even in the dry season, of which the creosote bush, 

 Covillea tridentata^ may be taken as an example. 



In addition, a fourth type may be added — the annuals, which 

 are really not xerophytic, growing both winter and summer in 

 the moist season. The seeds of the winter annuals do not ger- 

 minate in the summer rainy season, but wait until the following 

 winter; nor do the summer annuals develop in the winter. Thus 

 there are two quite distinct annual floras. 



The respiration of the succulent type as illustrated by cacti 

 was briefly spoken of at the end. It has long been known that 

 such plants absorb far more oxygen than they emit of carbon 

 dioxide, whereas in the typical plant the amounts are approxi- 

 mately equal. This is due to the fact that the oxidation of the 

 reserve carbohydrates is only partial and stops at the formation 

 of an acid, in this case apparently malic acid. This acid, how- 

 ever, is very unstable in light and during the daytime it is split 

 into simpler products, of which carbon dioxide is the final one. 

 Carbon dioxide arising from the photolytic eff"ect of light on the 

 accumulated acid is not however to be regarded as a true re- 

 spiratory by-product. One of the results of this action of light 

 is to cause a periodicity in the acidity of the tissues, which rises 

 at night and falls as the day progresses, to rise again as the sun 

 sinks. The amount of carbon dioxide must also vary, and indeed 

 on a bright and warm day the cactus, despite the fact that the 

 photosynthetic processes are active, actually gives forth this 



