620 



ECOLOGY 



that the glandular hairs of Drosera have been derived from water- 

 secreting glands through gradual specialization, but there is no good 

 evidence for this view; even the experimental method of attack is 

 likely to prove unavailing here. The theory of origin through natural 

 selection seems particularly inadequate, partly because in most cases 

 only highly developed organs can be of use, and partly because in all 

 cases the use appears too slight to account for preservation through the 

 operation of natural selection. 



8. LEAVES AS ORGANS OF SECRETION AND 

 EXCRETION 



General remarks on secretion and excretion. Secretion usually in- 

 volves the elaboration of new materials by specialized glands or glandu- 

 lar regions, whereas excretion involves 

 the elimination of waste by any organ. 

 While the products of secretion often 

 play an explicit role in subsequent ac- 

 tivities, there are many cases in which 

 no such role is known, so that it is im- 

 possible to regard all secreta as useful 

 and all excreta as useless substances. 

 Plants as a whole have less waste than 

 animals, probably because they utilize 

 simpler raw materials, taking in relatively 

 little useless matter. Plants also differ 

 from animals in that their excreta usually 

 accumulate in reservoirs or in dead or 

 inactive tissues instead of passing off, 

 although excreta are lost in large amount 

 through leaf fall and to some extent 

 through special organs. In any event, 

 the accumulation of waste products in 

 active cells is distinctly disadvantageous, 

 not only because some waste products 

 are toxic, but also because any such accumulations interfere with cell 

 activity. 



Water exudation. Hydathodes. When certain plants (as Tropae- 

 olum) are placed for some time in a moist chamber, liquid water is ex- 



FiG. 911. A surface view of 

 a nasturtium leaf (Tropaeolum), 

 showing large water stomata (w) 

 just over the terminal portion of 

 the vein (whose course is indicated 

 by broken lines), and numerous, 

 smaller air stomata (a) over the 

 mesophyll region of the leaf; note 

 the irregularity of stomatal orien- 

 tation, generally characteristic of 

 dicotyl leaves; considerably mag- 

 nified. 



