354 LECTURE XV. 



Kraus' observations on the occurrence of torsion in etiolated 

 internodes, a peculiarity to which Sachs first drew attention. 

 Kraus finds that the peripheral cortical cells, towards the end 

 of their growth, pass from the parenchymatous to the prosen- 

 chymatous form ; this is accompanied by considerable elonga- 

 tion, and it is at this time that torsion can be noticed on the 

 surface. Observations leading to the same conclusions had 

 been previously made by Braun on the development of 

 prosenchymatous wood-cells from the cambium. 



In connexion with these cases of torsion, we may notice a 

 curious phenomenon exhibited by roots which de Vries has 

 recently brought to light. It had been observed long ago 

 that the surface of the older parts of many roots (Red Clover, 

 Beet, Dipsacus Fullonum, Artichoke) is marked by transverse 

 or oblique wrinkles. De Vries finds that these wrinkles are 

 due to a change in form of the parenchymatous cells of the 

 root. When growth in length has ceased, these cells tend to 

 increase in breadth, and in so doing become shorter : this 

 causes the wrinkles in the cortical tissue, and also curvatures 

 of the vascular tissue. 



5. The Grand Period of Growth in Length. 



Now that we have acquired some notion of the structure 

 and properties of growing organs and of the mutual rela- 

 tions of the cells composing them when they are multicellu- 

 lar, we may go on to consider in greater detail the process of 

 growth itself. It has been already mentioned (p. 333) that 

 the growth of a cell is, generally speaking, limited to a par- 

 ticular period of its life. But the rate of growth is not 

 uniform throughout this period. At first the cell grows 

 slowly, then more and more rapidly until a maximum 

 rapidity is reached, and then the rapidity diminishes until 

 growth ceases altogether. This cycle is termed the Grand 

 Period of Growth. It is due entirely to causes inherent 

 in the growing cell, and though its regularity may be tem- 

 porarily interfered with by variations in external conditions, 

 yet it will on the whole assert itself in opposition to them. 



