774 KEMARKABLE PROPERTIES OF ROOTS. 



soil is made up of alternating places containing a larger and a smaller amount 

 of food-salts, and places which either retain water hadly or well. In one place 

 are situated nests of humus, in another sharp-edged stcmes, and it is only natural 

 that these ine(jualities and obstacles in the path pursued by the roots should not 

 be without eflect on them. As a matter of fact manifold conti'ivanccs are met with 

 for preventing the roots from, so to speak, blindl}' passing by favourable places in 

 the soil without making proper use of them. The fact that the tips of many roots 

 describe oscillations or nutations, not unlike those which are observed in twining 

 stems and in certain creepers, is an instance of such adaptation. Roots gi-owing 

 in soil are of course much more restricted in their movements by the pressure 

 of their environment than are the structures which circle rouml in the air, but 

 in the main the principle is the same in both cases. The path travelled by the 

 point of the growing root is most accurately depicted by a spiral line, and the most 

 important advantage obtained by following such a path lies in the contact of 

 the growing root with as large a portion of the soil as possible. A root growing 

 in a straight line would not touch half as many points as that following a spiral, 

 and since the likelihood that all the favourable spots will not be left on one 

 side increases with the number of points of contact, the spiral movement of 

 the roots may without hesitation be regarded as a contrivance for discovering 

 the best sources of food in the soil. We must, of course, not undervalue various 

 other advantages which are also obtained in this way, in particular, the greater 

 ease with which roots following a spiral line can bore their way into the soil, and 

 the better hold they obtain. 



Although the root follows a spiral line in its growth, it may nevertheless 

 maintain a straight direction on the whole; this is actually the case in water 

 and in a homogeneous and uniformly moistened soil. In soil differently constituted 

 and unequally moistened, however, a diversion takes place away from the side 

 where the conditions are unfavouralile to the root. This swerving maj^ be caused 

 by cold, dryness, by chemical conditions of the soil, and by pressure and injuries. 



It is well known that in the far north the ground remains always frozen below 

 a slight depth. During the short summer onlj- the superficial strata are thawed, 

 but below this the " perpetual ice " stretches to an immeasurable extent. A 

 relatively abun<lant vegetation develops on the thawed strata under the warm raj^s 

 of the sun, and in North America not only shrubs and low bushes but also colonies 

 of huge fir-trees grow up. The roots of these plants penetrate straight down- 

 wards and grow towards the centre of the earth; but as soon as they come 

 into the neighbourhood of the ice they bend aside, curve round, and continue theii 

 path only in the thawed stratum. The diversion is usually so striking that 

 the diverted portion is sometimes actually at right angles to the older part which 

 grew vertically dowuM^ards. 



The same thing happens when the soil is moist in some parts and dry in othere. 

 Here again the gi'owing roots seem to be repelled by the drj', inhospitable layers of 

 soil, and turn towards the neighbouring moister region. Tliis phenomenon has been 



