218 ABSORBING SYSTEM 



remove the soil-particles adhering to the latter by shaking and washing. 

 It will be found that the soil cannot be completely removed, except from 

 the actual root-tips, which hence have a smooth and whitish appearance 

 and from the portions which betray their age by their brown coloration 

 and in other ways. The zones bearing the absorbing tissue, on the 

 contrary, tenaciously retain a large proportion of the soil-particles, 

 which are indeed firmly glued to the root-hairs and thus appear 

 clothed in a coating of earth. 



The single superficial layer which represents the entire absorbing 

 tissue of the root, corresponds morphologically to the epidermis. The 

 cells of this layer are extended parallel to the long axis of the root. 

 They contain a moderately thick peripheral layer of protoplasm. Their 

 cellulose membranes are generally thin and soft, particularly in the 

 case of the outer walls, which constitute the actual absorbing surface. 

 These walls are further devoid of a cuticle, but instead usually possess 

 a coating of mucilage. The rest of the outer wall consists of cellulose 

 layers, which, according to Kroemer, sometimes undergo lignification or 

 become impregnated with protective substances of unknown composi- 

 tion. 



Among roots, one may recognise two stages in the specialisation of 

 the absorbing tissues, which differ from one another in respect of the 

 amount of surface that is made available for purposes of diffusion. 

 Some plants content themselves with the increase of surface which 

 can be obtained by augmenting the output of roots ; here the outer 

 walls of the absorbing cells are flat or slightly convex, so that the 

 absorbing region of the root is smooth. In other cases the enlarge- 

 ment of the absorbing surface depends chiefly upon the fact that the 

 absorbing cells are prolonged externally into so-called root-hairs. This 

 more advanced condition represents the normal type of absorbing tissue 

 in the case of roots. 



The simple form of absorbing tissue, in which root-hairs are absent, 

 suffices for the needs of such plants as grow under conditions that are so 

 favourable to the absorption of water and nutrient salts as to render 

 the exposure of a large absorbing surface unnecessary. Most marsh- 

 and water-plants are thus fortunately circumstanced. Hence the roots 

 of such species as Butomns umbellatus, Caltha palustris, Hippuris vul- 

 garis, Lemna minor, Nymphaat alba, Pistia Stratiotes, are entirely devoid 

 of root-hairs. Certain forms which grow in similar localities (e.g. 

 Scirpus si/lvaticus and Carex paludosa) are nevertheless provided with 

 root-hairs ; as a rule the plants in question can grow temporarily in 

 drier surroundings. 



The two stages of specialisation are, as a matter of fact, connected 

 by numerous intermediate steps. In this connection the varying 



