192 



BOTANY 



PART I 



Fig. 179.— Tip of a root-hair with adhering 

 particles of soil, (x circa 240.) 



particles of the soil, they die oft'. Above this advancing zone of hairs 

 the epidermis of the root becomes again completely divested of root- 

 hairs (Fig. 180). The older parts of 

 roots take no share in the process 

 of absorption. They envelop them- 

 selves with cork, increase their con- 

 ducting elements by growth in 

 thickness, and serve exclusively for 

 the transfer of the water absorbed 

 by the younger portion of the 

 roots. Even in the young roots the absorption seems principally con- 

 fined to the regions covered with root-hairs, or, when no root-hairs 

 are developed, to a corresponding zone of the 

 epidermis of the root. 



By the intimate union of the youngest 

 roots with the soil, they are able to withdraw 

 the minute quantity of water still adhering to 

 the. particles of earth, even after the soil 

 appears perfectly dry to the sight and touch. 

 There still remains, however, a certain per- 

 centage of water, held fast in the soil, which 

 the roots are not able to absorb. Thus, Sachs 

 found that the water left by a Tobacco plant, 

 which it could not absorb, amounted in cul- 

 tivated soil to 12 per cent, in loam to 8 per 

 cent, and in coarse sand to H per cent. 

 Plants may even obtain a certain quantity 

 of water from soil which is frozen hard or 

 from a block of ice. i 



^^^'^ 



The absorptive power of soil for water is diie to its 

 capacity to retain water by capillarity, so that it docs 

 not drain olf. Of the soils investigated by Sa(;hs, 

 cultivated soil retained in this way 46 per cent, loam 

 52 per cent, and sand only 21 per cent of water. 



If the development of the root system of a germinat- 

 ing Bean or Oak be observed, it will be found that the 

 growing root of the embryo at once penetrates the soil 

 and pushes straight downwards. Lateral roots are 

 then given off from the main axis, and, growing 

 either horizontally or diagonally downwards, penetrate Fio. 180.— Seedling of Carpinns 

 the earth in the neighbourhood of the primary root. ndulm. r, Zone of root- 

 These lateral secondary roots in turn develop other 

 roots, which radiate in all directions from them, and 

 so occupy and utilise the entire soil at their disposal. 

 The branching of the root system can proceed in this 

 manner until, within the whole region occupied by the roots of a large plant 

 there is not a single cubic centimetre of earth which is not penetrated and ex 



hairs near root-tip ; h, hypo- 

 cotyl ; liw, main root ; sic, 

 lateral roots ; I, V, leaf ; c, 

 epicotyl ; c, cotyledons. 



