VITALITY AND GEEMINATION OF SEEDS. 245 



lengths of the internal white tissue are exposed. On exam- 

 ining thin longitudinal sections of such roots the first 

 stage in the exposure of the subjacent unaffected portions 

 of the root is seen to be effected by a kind of sliding growth 

 of the internal regions beneath the outer. The inner 

 layers as they go on growing stretch and split the outer 

 brown layer or layers and the tissue thus exposed being 

 released from the pressure of the outer layers which 

 formerly compressed it bulges somewhat outwards. In a 

 short time what is now the outermost living layer of the 

 cortex assumes the regular outline and apparent function 

 of the epidermis or " epiblema " and may form root hairs. 

 This power of adaptive modification shown by the exposed 

 cortex is due to the inner cortex of these regions being 

 still capable of growth and hence more or less meristematic 

 and it is shown more markedly and rapidly by the exposed 

 regions nearer the apex than by those further removed. 



The greater the growth of any particular part of the root 

 the more widely will the rings of browned tissue be 

 separated from each other. Hence the first crack is 

 formed just behind the root cap and in a short time a 

 number of rings are formed over the terminal part of the 

 root. The following is an illustrative example : 



In one day after the resumption of growth ; the distance 

 from the root cap to the first ring was 5*8 m.m. ; from 

 1st to 2nd ring 1 m.m., from 2nd to 3rd ring '5 m.m., and 

 from 3rd to 4th ring '1 m.m. In two days the first region 

 had increased to 1*54 m.m., the second to 1*9, the third 

 to *6, the fourth to *16 whilst the distance from the fourth to 

 a fifth ring was "08 m.m. The whole of the rest of the 

 length of the root did not elongate at all and remained 

 with an unbroken brown covering. The experiments thus 

 show very clearly the restriction of growth in length to 

 the apical region and more especially to the region a little 



