198 



BOTANY 



Fio. 185. Tip of a ruot-liair witli 



I>art ides of soil, (x circa -J40.) 



ll' the development of the root system of a germinating Beau or Oak be observed, 



it will be found that the growing root of the embryo at once penetrates the soil and 



I pushes straight downwards. Lateral roots 

 are then given off from the main axis, and, 

 growing either horizontally or diagonally 

 downwards, penetrate the earth in tin- 

 neighbourhood of the primary 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 branch- 

 ing 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 exhausted by them. The 



system of branches of one root system often has an 



astonishingly great total length, and may amount to 



a kilometre in an annual cereal, and to several kilo- 

 metres in a well-developed Cueurbita. 



All plants do not form a deep-growing tap-root like 



that of the Oak, Silver Fir, Beet, Lucerne, etc. ; some 



confine themselves to utilising the superficial layers 



of the soil by means of a thickly branched lateral root 



system (Pine, Cereals). The agriculturist and forester 



must, accordingly, take into consideration the mode 



of branching and growth of the roots of a plant just 



as much as the habit of growtli of its aerial portions. 



Plants which make use of different layers of soil 'may 



be safely cultivated together in the same soil, and 



succeed one another in the same ground. For similar 



reasons, in setting out trees along the borders of fields, 



the deep-rooted Elm should be preferred to the Poplar. 



whose roots spread out near the surface. 



Desert or xerophilous plants, according to the ob- 

 servations of YOLKENS, send out deeply penetrating 



roots, which only branch profusely on reaching depths 



where they find water. When grown in moderately 



dry soil the growth of the roots of oats is greater than 



when the soil is damp. 



In order to secure a still more intimate 

 contact with the particles of the soil, there 

 are produced from the surface of roots small, 

 exceedingly numerous, and fine cylindrical 

 bodies, which penetrate the smallest inter- 

 stices of the soil, and fasten themselves so 

 closely to its smallest particles as to seem 

 actually grown to them (Fig. 185). These 

 ultimate branches of the root system, which 

 discover the very smallest quantity of moisture, and seek out the 

 most concealed crevices in the search for nourishment, are the ROOT- 



FIG. 180. Seedling of i orpimu 

 Betulus. r, Zone of root- 

 hairs near root-tip ; h, liypo- 

 cotyl ; hw, main root : .-or. 

 lateral roots; I, I, lent: >. 

 epicotyl ; r, cotyledons. 



