THE STRUCTURE OF THE ROOT 9 



If we imagine a finger covered by a thimble, which adheres 

 to the finger by its internal surface, we have a rough but 

 pretty accurate illustration of a root-apex with root-cap. 



Beneath this root-cap lie the tissues of the root-tip, and in 

 different arrangement according to the different genera. In 

 the majority of cases a central group of meristeni cells can be 

 distinguished (Fig. i PI), which give rise to the new portions 

 of the central vascular cylinder. This central portion is 

 surrounded by a number of layers of cells (P) which give rise 

 ■to the cortex. A single layer of cells (D) covers in the cortex 

 over its whole extent, and will form the epidermis and the 

 absorbing cells or root-hairs. At the very extreme point of 

 the root-tip these tissues all converge into a group of cells, 

 ■which is the seat of formation of all these layers, and is also 

 the point of connection between the root-tip and the root-cap. 

 The root itself, therefore, and the root-cap have in reality the 

 same point of origin, namely, the extreme apex of the root ; and 

 in the generality of cases, in dicotyledonous plants, we may look 

 upon the root- cap as merely a production or proliferation of the 

 youngest epidermal layer of the root itself. 



This protuberance of the root is renewed from its inner 

 layer, i.e., from its point of connection with the root itself, 

 while its outer and therefore oldest cells {a) gradually lose 

 their contents, allow their cell-wall to break down into muci- 

 lage, and in the end separate from each other and break down. 

 During this process carbonic acid is given off, and becoming 

 absorbed by the water, it causes mineral salts, such as phosphate 

 of lime, which are insoluble in pure water, to become dissolved. 



This we may look upon as one of the functions of the root- 

 cap. The salts which the decay of the older root-cap cells 

 causes to be dissolved are absorbed by the inner younger cells 

 and conducted to the centres of growth. To that extent, 

 therefore, the root-cap may be considered an organ of absorp- 

 tion ; its chief function, however, is that of a protective cap. 

 We can readily understand what obstacles a continuously 

 elongating root-tip has to encounter in its efforts to force a 

 passage through the firm soil. It requires the application of 

 special forces and special means of protection. The force 

 which enables the root to bore its way into the soil is the 



