502 DEPENDENCE OF PLANT FORM ON SOIL AND CLIMATE. 



Plants routed in the mud of a river-bed, the stems and leaves of which are 

 surrounded by rapidly-flowing water, must possess corresponding strength if 

 they are not to be torn. In comparing two plants of the same species, the one 



growing in the still water of a deep lake, the other in a rapidly-flowing stream 

 it will be noticed that the walls of the superficial cells of the latter have become 

 strongly thickened, and that strong bundles of bast-fibres have developed in the 

 cortex of the stem, while in the former only the weakest traces of bast-fibres 

 can be seen. The extraordinary length of stem, petiole, and leaf-blade is also 

 very surprising in plants which grow in rapid water. The Pondweed Potamogeton 

 fluitans, the Rushes Jvmcus lamvprocarpus and xupinus, the Grasses Agruxti* 

 stolon iff rtt and Glyceria fluitans are very instructive examples. A plant of 

 the last-named Grass growing on damp soil on the edge of a stream over the 

 water had linear, bluntly-pointed leaves, whose sheaths were on the average 15 cm. 

 long, the blades 23 cm. long and 8"5 mm. broad. After this plant had been sub- 

 merged under rapidly-flowing water in the following year, leaves unfolded, which 

 tapered gradually to a point, with a sheath having a mean length of 47 cm., and 

 blades 73 cm. long but only 5 mm. broad. The blades produced in running 

 water were three times as long and actually rather narrower than in the air. 

 There was no difference in the number of strands traversing the blade, but they 

 were nearer to one another than in the aerial leaves. The Arrow-head iSiigittorin 

 sagittifolia), which usually grows on the muddy bottom of shallow lakes, raising 

 its leaves above the still water, has gained its name from the likeness of its 

 leaf-blade to an arrow. If it is planted in the bed of a rapid stream so that 

 the leaves during their development are exposed to a vigorous current, the 

 leaf-blade is almost entirely suppressed. What still remains has the form of a 

 spade, but not infrequently all trace of lamina is wanting. The petiole, however, 

 lengthens to 70 cm., and forms a limp, flat, pale-green ribbon 1-2 cm. broad, 

 which might easily be mistaken at first sight for the leaf of Vallisneria. 



Another remarkable change which is effected by submerging growing plants 

 is the non-development of the epidermal structures called hairs, so that the leaves 

 and stems of submerged plants always appear smooth. The suppression of hair- 

 structures is very noticeable in the aquatic variety of Polygonum, amphibium. 

 In aerial plants of this species the leaves have short petioles, are lanceolate in 

 shape, and are covered thickly with short hairs, which are rough to the touch: 

 while the aquatic plants have long-stalked, broadly-linear leaves completely smooth 

 on both sides. 



The humidity of the atmosphere has a marked effect on the form of land 

 plants. Transpiration, which is so deeply concerned in all the vital processes 

 is carried on very slowly in air which is almost or quite saturated with water 

 vapour. If plants of a species which usually grows in dry air come into a humid 

 atmosphere, they must be furnished with means for aiding evaporation. On 

 the other hand, plants which grow in dry air must be protected against excessive 

 transpiration. The aids and protective measures were so minutely described 



