THE CONDITIONS OF PLANT LIFE 23 



which exists between the lowest members of the two 

 great organic kingdoms. This power of free locomotion 

 is also found in some of the reproductive cells of most 

 plants except the highest ones, and even in a few of 

 the seed plants these have recently been discovered, 

 the male reproductive cells having the form of ciliated 

 spermatozoids which are actively motile. This power 

 of spontaneous locomotion is finally lost, and in nearly 

 all the seed plants is completely absent, although even 

 here the plant exhibits more or less marked movements 

 of various kinds. Some of these movements are appar- 

 ently spontaneous, such as the revolution of the apex 

 of the growing stem and root, and of tendrils, but oth- 

 ers are influenced directly by external agencies, light, 

 moisture, contact, and gravity. The spontaneous move- 

 ments of the growing apex of many plants is called 

 nutation, and is apparently quite independent of ex- 

 ternal agencies. The effect of light in plants is well 

 known. The response of actively growing plants to 

 this stimulus is often very rapid, although the exact 

 mechanism of these movements is not entirely clear. 

 Occasionally plants are negatively heliotropic, i.e. they 

 grow away from the light, as is seen in the common ivy. 

 Light is, in most cases, necessary for the formation of 

 chlorophyll, as well as for the performance of photo- 

 synthesis in those cells which contain the chlorophyll. 

 Doubtless the cause of the movements of plants is 

 largely due to the direct effect of light upon the sen- 

 sitive protoplasm of the cells of the motile parts. 

 This is indicated by the activity of naked swarm- 

 spores and plasmodia (masses of naked protoplasm 

 found in certain low organisms, such as the Slime- 



