THE ROOT. 17 



vascular cylinder, and composed of cells which differ from those 

 of the growing-point chiefly in their elongated shape. Cambium 

 and increase in thickness will be more fully spoken of on pp. 31 

 and 36. 



We are now in a position to make a somewhat wider classifica- 

 tion of tissues. They may be grouped as follows : — 



I. Formative Tissue or Meristem. 



Composed of actively-dividing cells, with thin cellulose walls 

 and abundant protoplasm. 



1. Primary Meristem. Making up growing-points and the 



whole of very young structures. 



2. Secondary Meristem or Cambium. Forming layers bounded 



internally and externally by permanent tissue. 

 II. Permanent Tissue. 



Composed of cells not in a state of active division (though they 

 may retain the power of dividing) and cell-derivates. 



1. Epidermis. An external layer of flattened cells, to which 



hair- structures may be attached. 



2. Ground-tissue. Largely composed of parenchyma. 



3. Vascular system. Composed of cells and vessels. 



PHYSIOLOGY. 



The root of a land plant is protected to a great extent by its 

 position in the soil, the root-caps protecting its delicate growing- 

 points from injury. The epidermis at first protects from evapo- 

 ration, and, in older examples, from which parts external to the 

 vascular cylinder have peeled off, its place is taken by layers of 

 cork (cf. p. 40). 



The root serves as an organ of support to the stem and leaves. 

 It is often firmly fixed in the soil by numerous branches, and the 

 nature of the vascular cylinder is such as to give firmness. The 

 root is essentially a vegetative organ, i.e., is concerned with main- 

 taining the life of the individual, and consequently plays a very 

 important part in Nutrition, since, in land plants, all the food, 

 with the exception of carbon dioxide, is absorbed by it. The soil 

 is made up of variously- sized particles, with spaces between them, 

 that contain, according to circumstances, more or less water with 

 various substances dissolved in it, the part not thus occupied 

 being filled with air. Part of this water is readily drained off, 

 and has been termed free water, while the remainder is in the 

 form of films that surround the particles. These films, consti- 

 tuting the hygroscopic water, are the part which furnishes the 

 plant with nutriment, and may be looked upon as a natural food 

 solution. The absorption is effected by the young epidermic cells 



B 



