24 EVOLUTION OF PLANTS 



moulds, Fig. 4), when exposed to light, the green 

 swarm-spores of algae almost always being very quickly 

 attracted to the light. 



Geotropism, or the movements induced by gravity, 

 are either negative or positive. In the higher plants, 

 the aerial parts, especially the stems, are usually nega- 

 tively geotropic, i.e. grow upward, the roots positively 

 geotropic. 



The marked movements of the sensitive plant (Mi- 

 mosa), as well as the similar movements especially of the 

 floral parts of many plants, and the so-called " sleep move- 

 ments " of such leaves as the locust, clover, and many 

 others, are connected with changes in the turgor of the 

 cells of special parts of the motile organs. These 

 movements are undoubtedly, like the movements due 

 to simple heliotropism, intimately associated with the 

 sensitiveness of the protoplasm, and are induced by a 

 variety of stimuli, such as shocks, light, and electricity. 



The absence of locomotion in the higher plants is in 

 large measure due to the investment of the cells with a 

 firm membrane, and it is only when the protoplasm 

 escapes from the cell, as in the case of swarm-spores 

 and spermatozoids, that the primitive power of locomo- 

 tion is regained, and recalls the possession of this 

 animal character by the ancestors of all the higher 

 plants. 



It is a popular fallacy that plants and animals supple- 

 ment each other in their method of respiration. It is 

 not necessary to remind the botanist that this mistake 

 is based upon a confusion of terms. During the process 

 of carbon assimilation in the green cells, there is a large 

 amount of free oxygen liberated — much in excess of 



