. A textbook of botany for colleges and universities ... Botany. 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. LEA
. A textbook of botany for colleges and universities ... Botany. 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 r61e 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)y showing large water stomata (w) just over the terminal portion of the vein (whose course
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
