92 



PLANT RELATIONS. 



As is well known, when a seed germinates the tip that is to 

 develop the root turns towards the earth, even if it has 

 come from the seed in some other direction. This response 

 to gravity by the plant is known as geotropism. Another 

 directing influence is moisture, the response to which is 



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Fig. 87. A section through the leaf stalk of a yellow pond lily (Nvphar), showing the 

 numerous conspicuous air passages (*•) by means of which the parts under water 

 are aerated ; h, internal hairs projecting into the air passages ; r, the much 

 reduced and comparatively few vascular bundles. 



known as hydrotropism. By means of this the root is di- 

 rected towards the most favorable water supply in the soil. 

 Ordinarily, geotropism and hydrotropism direct the root 

 in the same general way, and so reinforce each other; but 

 the following experiment may be arranged, which will 

 separate these two influences. Bore several small holes in 

 the bottom of a box (such as a cigar box), suspended as in- 

 dicated in Figure 85, and cover the bottom with blotting 

 paper. Pass the root tips of several germinated seeds 



