BOTANICAL INTBODUCTION 39 



Eoot and Stem. 



For our present purposes, a plant may be considered as 

 consisting of root, stem, branches, leaves, flowers, and fruit. 

 The root" is developed from the rootlet or radicle of the 

 embryo, and serves two purposes. It fixes the plant in the 

 soil, and absorbs water and salts in solution by means of 

 minute thin- walled hairs on the rootlets. The stem supports 

 the leaves and branches. The crude sap ascends through it to 

 the foliage at the tips of the highest twigs by a process or 

 processes, even now not well understood. Here the water is 

 evaporated, and any salts that it still contains are left behind 

 in the leaves and shoots. The stem and branches therefore, 

 as well as supporting the plant, enable the sap to ascend and 

 descend, and thus provide communication between different 

 parts of the tree. 



THE LEAF. 



The leaf is the chief organ by which the plant nourishes 

 itself. Air enters it by minute pores (stomata, singular stoma), 

 and brings with it a small percentage of carbon dioxide. The 

 leaf retains the carbon, giving up most of the oxygen. With 

 the carbon, water, some oxygen and the salts obtained from 

 the soil, the plant is built up by marvellous and little known 

 chemical reactions. The leaf, like all other parts of the plant, 

 consists of layers of very small cells. The first solid product 

 to appear in certain highly specialized cells as a result of the 

 absorption of carbon dioxide, is starch. This consists of the 

 three elements, carbon, hydrogen and oxygen, and is produced 

 only in the presence of the green colouring matter (chlorophyll) 

 by the action of sunlight. The process by which starch is 

 produced is termed assimilation. 



A description of the structure of a typical dicotyledonous 

 leaf, will enable us to understand this function better. 



*(The root is considered rather more fully under Avicennia.) 



