^74 HOW CROPS GROW. 



short exposure to dry air, or rot, if long immersed in 

 water. Ma,ny aquatic plants, on the other hand, speed- 

 ily die when their roots are removed from water, or from 

 earth saturated with water, and exposed to the atmos- 

 phere or stationed in earth of the usual dryness. 



Air-roots are not common except among tropical plants 

 or under tropical conditions of heat and moisture. In- 

 dian corn, when thickly planted and of rank growth, 

 often throws out roots from the lower joints of the stem, 

 which extend through the air several inches before they 

 reach the soil. The same may be observed of many com- 

 mon plants, as the oat, grape, potato, and buckwheat, 

 when they long remain in hot, moist air. The Banyan- 

 tree of India sends out from its branches, vertically, 

 pendants several yards long which penetrate the earth 

 and there become soil-roots. 



On the other hand, various tropical plants, especially 

 Orchids, emit roots which hang free in the air and never 

 reach the earth. In the humid forest ravines of Madeira 

 and TenerifEe, the Laurus Ganariensis, a large tree, 

 sends out from its stem, during the autumn rains, a pro- 

 fusion of fleshy air-roots, which cover the trunk with 

 their interlacing branches and grow to art inch in thick- 

 ness. The following summer they dry away and fall to 

 the ground, to be replaced by new ones in the ensuing 

 autumn. (Schacht, Der Baum, p. 172.) 

 , A plant, known to botanists as the Zamia spiralis, not 

 only throws out air-roots, c c. Fig. 44, from the crown of 

 the main soil-root, but the side rootlets, h, after extend- 

 ing some distance horizontally in the soil, send, from the 

 same point, roots downward and upward, the latter of 

 which, d, pass into and remain permanently in the air. 

 a is the stem of the plant. (Schaoht, AnatMiie der 

 OewUchse, Bd. II, p. 151.) 



The formation of air-roots may be very easily observed 

 by placing water to the depth of half an inch in a tall 



