280 HOW CROPS GROW. 



unless profusely watered or shielded from evaporation. 



The air-roots of Orchids, which never reach the soil, 

 have a peculiar spongy texture and take up the water 

 which exists as vapor in the air, as shown by the experi- 

 ments of linger, Chatin, and Sachs. Duchartre's inves- 

 tigations led him to deny their absorptive power. (Ele- 

 ments de Botanique, p. 216.) In his experiments made 

 on entire plants, the air-roots failed to make good the 

 loss by evaporation from the other parts of the plant. 



It is evident from common observation that moisture 

 is the condition that chiefly determines root-develop- 

 ment. Not only do all seeds sprout and send forth roots 

 when provided with abundant moisture at suitable tem- 

 peratures, but generally older roots and stems, and 

 fleshy leaves, or cuttings from these, will produce new 

 rootlets when properly circumstanced as regards moisture, 

 whether that moisture be supplied by aid of a covering 

 of damp soil, wet sand or paper, by stationing in humid 

 air, or by immersion in water itself. 



Root-Excretions. It was formerly supposed that 

 the roots of plants perform a function of excretion, the 

 reverse of absorption that plants, like animals, reject 

 matters which are no longer of use in their organism, 

 and that the rejected matters are poisonous to the kind 

 of vegetation from which they originated. De Candolle, 

 an eminent French botanist, who first advanced this doc- 

 trine, founded it upon the observation that certain plants 

 exude drops of liquid from their roots when these are 

 placed in dry sand, and that odors exhale from the roots 

 of other plants. Numerous experiments have been in- 

 stituted at various times for the purpose of testing this 

 question. Noteworthy are those of Dr. Alfred (iydr 

 (Trans. Highland and Ayr. Soc., 1845-47, pp. 273-92). 

 This experimenter planted a variety of agricultural plants, 

 viz., wheat, barley, oats, rye, beans, peas, vetches, cab- 

 bage, mustard, and turnips, in pots filled either with 



