252 PLANT-LIFE 



co-operate in building up " the plant," and certain of 

 them are told off for particular work. All are chemists, 

 but some specialize in one way and some in another. 

 There are carbon-assimilators in the green leaf, workers 

 in salts in the root hairs, sap-carriers in the vascular 

 tissues. In such a community the business of sex is 

 not transacted promiscuously among any of the cells, 

 but some are specially developed and told off for the 

 duty. 



It is, then, the living protoplast that promotes growth, 

 and in order to do so must be nourished with suitable 

 food. The requirements of a plant are those of the 

 protoplasm which constitutes it. We shall now decide 

 the nature of these requirements. 



We have every reason to believe that living proto- 

 plasm came into existence in water, and that the earliest 

 life-forms, both animal and vegetable, flourished in that 

 medium. To say that land plants, now far removed 

 from their aquatic ancestry, cannot live without water, 

 is stating common knowledge, for plants insufficiently 

 supplied with water droop. If the drooping individuals 

 are furnished with water before it is too late, they revive, 

 and become erect and tense; and if the supply is with- 

 held too long, they wither and perish. But it is not 

 realized by the unscientific observer that the water 

 required by land plants is really demanded by the proto- 

 plasts that form them and control their existence. 

 Protoplasm cannot operate in what the Scotsman calls 

 " drouth." If moisture is not sufficient, it must sus- 

 pend activity; and if desiccation occurs, it must die. 

 In fact, in spite of its conquest of the land, protoplasm 

 has never rendered itself independent of water. If it 



