32 



PLANT HABITS AND HABITATS IN THE 



I is accompanied by a change in the "relative transpiration," or tran- 

 spiration power (relation between rate of transpiration and rate of 

 evaporation) . Under dry conditions the t/e ratio is greater by day, but 

 under moist conditions it is greater by night (Shreve, E. B., 1915: 79). 

 Vxhe fluids of desert plants have a high concentration, as determined by 

 \ Fitting (1911:209), Lawrence, Gortner, and Harris (1916:1). The 

 I concentration, of the juices varies in relation to local environmental 

 Jconditions. \ It is least in the arroyos and greatest in the salt spots^x 

 T'or example, an average of eight determinations of the density of the 

 juices of plants from the latter habitat gave 37.1 atmospheres. Table i 

 9 summarizes these results. 



Table ^.—Oimoiie pressure, in atmospheres, of various growth-forms in five habitats of tht 

 Tucson region (Harris, 1916 : 81). 



Richards (1918: 64) finds that a certain species is more or less suc- 

 culent when growing under dry conditions, whereas the typical forms, 

 imder moist conditions, have thin leaves. In every instance the more 

 Bucfcuient form developed less acid than the form less succulent. 



frhe ojyness of /the atmosphere works immediately to influence the 

 mation\if a ra-eater amount of cellulose and a lesser amount of 

 rch (MacDmieal and Spoehr, 1918^:247). Thus the polysacchar- 

 s are Gon-venjfe- into anhydrides or wall material under conditions 

 aridity, or in sbtjculeait species, polysaccharides are converted into 

 itosans or niicilages.\ These changes, particularly the last, are of 

 great physiological importance to the species, inasmuch as the "imbi- 

 ' bition " capacity of the polysaccharides is small. Their transformation 

 from this form into that of the pentosans gives the increased capacity 

 (of imbibition) characteristic of the pentosans, sottnat without any 

 addition of material to a cell, but simply by the loss of water, a change 

 takes place by which the cell is capable of absorbing and holding vastly 

 gl^gi^piopoytions of water. 

 ^ Low water-content of certain cacti results in a condition of general 

 reversion of carbohydrates to polysaccharidesi The simpler sugars, or 

 monosaccharides, decrease in amount in the plants as the water-content 

 is reduced. With continued low water-content\the pentosans increase 

 decidedly (Spoehr, 1918:62). / 



,t;::=^It would appear, therefore, that dryness ^i itself may profoundly 

 \ modify the chemical nature of plants exposed to its' influence and it may 

 \ lead, as indicated above, on the one hand to formation of wall material, 



