THE STEM 157 



into three groups, to which different functions are 

 mostly, if not exclusively, natural. These three groups 

 or categories are the following : cells proper, fibres, and 

 tubes. In the cells the processes of nutrition take place, 

 i.e. the formation and transformation of the nutrient 

 substances. They contain the chlorophyll ; in them the 

 stores of albuminous matter are deposited, also starch, 

 sugar, crystals of mineral salts, and so on. The cells are 

 the laboratories and warehouses of the plant. Fibres 

 serve mainly for mechanical purposes, their contents 

 being of no value whatever. Their chief end is served 

 by their elongated form and by their walls, which are 

 sometimes so much thickened as to entirely fill the cavity 

 (as is shown in fig. 42, 5). Recent investigations have 

 shown that the material out of which these mechanical 

 elements are formed, as well as their structure and 

 especially their distribution in the stems, render them 

 wonderfully well adapted to their function, which is to 

 impart to the various parts of the plant the necessary 

 firmness and elasticity with all possible economy of 

 building material. Investigation has proved, for instance, 

 the remarkable fact that the material out of which 

 these fibres are formed is almost as strong as iron in 

 some respects, and that the fibres are distributed accord- 

 ing to the laws of engineering. The tubes, belonging 

 to the third category, serve mainly as sap conductors. 



Let us now see how these elements, so various in 

 structure and function, are distributed in the plant. 



Cells proper form a connective or fundamental 

 tissue, i.e. the foundation of all organs connecting 

 all their various parts together, while fibres and 

 tubes (or vessels) are grouped together in strands, 

 called vascular bundles, which pass through this funda- 

 mental tissue. We see this best of all in leaves. The 

 middle part of the leaf between the upper and under 

 epidermis is occupied, as we already know, by the 

 fundamental tissue, through which run veins or nerves. 



