126 GENERAL BIOLOGY 



the trunk being confined to the comparatively narrow 

 layer between this and the bark. 



In the leaves the skeleton assumes the form of 

 branching " ribs " or " veins." In some groups, 

 e.g. the grasses, these v are single and unbranched ; 

 in others they form a network. 



Circulatory System. In plants, as in animals, in 

 those species which are of such a size that substances 

 do not diffuse readily through their tissues, there is a 

 system of tubes which permit the circulation of 

 liquids from one place to another. The movement 

 of the circulating liquids is due to mechanical 

 agencies external to the protoplasm. There is, of 

 course, nothing corresponding to a heart. The tubes, 

 which are of various sorts and positions, often occur 

 in bundles. They are formed by the coalescence of 

 cells, end to end, and the subsequent dying out of the 

 living contents. The prime factor in causing the 

 ascent of water in the plant tissue is the " transpira- 

 tion " or evaporation of water from the leaves. 

 Just how this operates is still a complicated and un- 

 solved problem. A result of such movements of 

 liquid is the transportation of food substances to 

 various portions of the plant-body. 



Alimentary System. Algse derive their food from 

 the surrounding medium, and their tissues are cor- 

 respondingly soft and permeable. There is for this 

 reason no localized area for the alimentary processes. 

 In land plants, on the other hand, there is a twofold 



