GROWTH OF ROOTS IN THE SOIL. 



17 



also in general at the same time moister, from which 



A 



.^ 



from the light, which 

 alone new roots arise. 



A root having in this way penetrated into the earth, and grown forwards 

 perpendicularly within it, lateral roots aiise progres- 

 sively from above downwards, in the order stated. 

 These also are geotropic, and sensitive to contact and 

 to moisture, but in a different degree from the tap-root : 

 their geotropism especially is different; the lateral 

 roots are caused by the influence of gravitation 

 to grow, not vertically downwards, but obliquely, or 

 even horizontally. However difficult it may be to 

 conceive this varying sensitiveness of the different 

 roots of a system, thus much is obvious, that 

 the entire root-system is compelled to grow into the 

 earth in all directions by it. At a later period, when 

 we are particularly concerned with the irritability of 

 plants, I can speak in detail concerning the pro- 

 perties of roots here mentioned: the point now is 

 simply to establish the existence of these properties, 

 and to insist upon their purpose for the life of the 

 plant. 



In this connection, however, there are yet a few 

 points of special interest: in the first place stress is 

 to be laid upon the fact, that in roots growing into the 

 earth, the portion which is becoming elongated behind 

 the growing point is very short ; even in vigorous 

 tap-roots only 8-10 mm. long, and in thin lateral 

 roots often only 2-3. This appears remarkable, when 

 one considers that in aerial leaf-shoots and flower- 

 scapes the elongating portion is commonly many centi- 

 metres long. In the shortness of the root, however, an 

 arrangement is afforded which renders the progress of 

 the apex of the root within the earth considerably 

 easier, since the whole force of the push by which 

 the growing point is driven forward is concen- 

 trated immediately behind it. In the long aerial roots 

 of tropical Aroids, and others, where this mechanical 

 arrangement is unnecessary, I found, accordingly, a 

 much more considerable length of the elongating 

 region. 



Fundamentally, however, the roots, as we have 

 hitherto learned to understand them, are only the bearers 

 of the proper organs for the absorption of food, namely, 

 the root-hairs. These are exceedingly delicate-walled, 



narrow tubes, some millimetres in length and a few y^^ ths of a millimetre in diameter, 

 which grow forth from the surface of the roots in very large numbers. If roots 

 [3] 



Fig. 9.— Diagram of the primary root 

 and lower part of the shoot-axis of a seedling 

 of Piszem satiimnt (Pea). Cot cotyledons ; 

 a, d, c, d, e transverse sections at the vari- 

 ous heights ; P (everywhere) the phloem of 

 the vascular bundles ; vih a secondary root 

 (behind); vis secondary roots at the sides 

 of the seedling ; Bk the first leaf (behind) ; 

 vll tertiary roots. 



