20 BOTANY. [CHAP. i. 



case of Pleurococcus and its allies that have established 

 themselves on dry land, active life can only proceed 

 when the cells are supplied with the requisite amount of 

 water, under other conditions the plant passes into a 

 dormant or resting state. Certain members of the 

 Hepaticce, small moss-like plants, were amongst the first 

 to solve the problem as to the method of preventing 

 desiccation by introducing the idea of division of labour, 

 brought about as follows : In the species of Marchantia, 

 popularly known as liverworts, the vegetative portion is 

 prostrate and in contact with the damp ground by the 

 whole of its under surface, the outer layer of cells of the 

 upper surface undergoing a change by which they are ren- 

 dered waterproof, and thus form a protective layer that 

 prevents evaporation of water from the tissues of the 

 plant into the surrounding dry air. This superficial 

 layer of modified cells constitutes the epidermis, and 

 proved so effectual that the idea has never been super- 

 seded, and is still the means of preventing undue loss of 

 moisture employed by the most highly differentiated 

 flowering plants. The epidermis is furnished with minute 

 apertures called stomata which admit of an interchange 

 between the gaseous contents of the plant and those of the 

 atmosphere, details of which will be given at a later stage. 

 A second important modification of plant structure 

 necessitated by the previous development of an epidermis 

 was the formation of fibro-vascular bundles, or modified 

 portions of the original cells of aquatic plants into elon- 

 gated, thick- walled cells pointed at the tips, or elongated 

 cells of large diameter having the walls strengthened by 

 internal ribs arranged in various ways; these modified 

 cells are usually arranged in groups, and in the higher 



