72 



A PRIMER OF BIOLOGY 



protoplasm to light has been given in the case of the 

 movements of zoospores (p. 64). 



Water is as important a factor in the life of the 

 green plant as light, and it is therefore obvious that 

 it is of the utmost value, to the root especially, that 

 it should be sensitive to its presence ; roots, as a 

 matter of fact, grow towards water. Hence the 

 frequency with which drain-pipes are clogged up by 

 the intruding roots of plants living in the vicinity. 

 A very interesting and at the same time simple 

 experiment serves to demon- 

 strate the predominant effect 

 of water as a stimulus over 

 gravity. Remove the bottom 

 from a cigar-box and replace 

 it by one made of wide meshed 

 wire netting, floor the inside 

 with wet bog moss and plant 

 in it some peas or other seeds 

 (Fig. 29). In a few days the 

 roots, in obedience to the 

 stimulus of gravity, will have 

 grown through the wire netting 

 and into the air below. Finding, however, that the 

 air is less moist than the moss above them, they 

 change their direction of growth and bend back again 

 into the box, thus showing that the hydrotropic 

 stimulus is more vigorous and effective than the 

 geotropic stimulus. 



Let us briefly consider, in conclusion, the influence 

 of some chemical stimuli on plants. One of the com- 

 monest weeds in our rivers and canals is an American 

 aquatic plant, known as Elodea. If some of the young 

 leaves of this plant be placed under the microscope 

 it will be seen that the chloroplasts and other contents 



FIG. 29. Hydrotropism. 



