THE DEPTH IN THE SOIL AT WHICH PLANTS OCCUR. 489 



active, living cells undergo a shortening in the 1 ongitudinal 

 direction combined with a marked radial extension. At the 

 same time — and this is an essential factor — there is an absorption 

 of water by these cortical cells whose shortening in the one 

 direction is more than compensated by expansion in the other. 

 The root, therefore, though becoming shorter, may in the aggre- 

 gate be bulkier than it was before, unless it happens, as some- 

 times is the case, that a certain number of layers of cells in the 

 outer cortex become flattened and obliterated. The effect of the 

 shortening cortex is a very definite one upon the other parts of 

 the root. The central cylinder of the root, in which are located 

 the vascular strands, is forcibly compressed, and in cases in 

 which it is resistent it may assume a sinuous course. The 

 periphery of the root, i.e. the piliferous layer, and the imme- 

 diately subjacent layers of the cortex being by this time dead, 

 and their walls in part suberised, cannot participate in the 

 general contraction. It is this portion which, as a consequence 

 is thrown into transverse ridges and furrows, giving the charac- 

 teristic appearance of contracted roots. 



In addition to these the principal phenomena of root-short- 

 ening, a transverse striatum of microscopic dimensions is met 

 with in the endodermis and in the layer of cortex immediately 

 beneath the piliferous layer. This striation is the expression of 

 minute foldings of the suberised walls peculiar to these layers, 

 and according to Rimbach arises only in roots which have 

 entered on the contractile phase. 



The amount of actual shortening is very different in different 

 plants. It may be only 5 to 10 per cent, of the total length involved, 

 or in other cases may attain to 80 or 40 per cent., or even more. 



The special feature associated with root-contraction, with 

 which we have more particular concern here, is the manner in 

 which it leads to the drawing of the whole plant down into the 

 soil. As a root develops it continually penetrates deeper into 

 the soil and comes into intimate contact by means of its root- 

 hairs with the particles of the soil, and as the apical develop- 

 ment continues it becomes in time very firmly fixed. When, 

 now, contraction supervenes in the older parts, tensions arise 

 which lead, in the case of roots normally fixed, to a gradual 

 pulling down of the whole plant into the soil. So considerable 

 is this tension that if the root be cut, a space 2 or 3 milli- 



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