GROWTH. 351 



sure upon the wall between a parenchymatous and a vascular 

 cell is, whilst the vascular cell is young and turgid, the same 

 on both sides, but, as the vascular cell becomes gradually 

 differentiated so as to form a segment of a vessel, it loses its 

 protoplasmic contents, and with them its capacity for becom- 

 ing turgid : the wall is now subject to pressure from the side 

 of the parenchymatous cell only, and those portions of it 

 which have remained thin and extensible, the pit-membranes, 

 yield to the pressure and grow out into tyloses in the manner 

 described above. The fact that the parenchymatous cells 

 begin to grow at those points at which the pressure upon 

 them is removed, proves that their growth is hindered by the 

 pressure to which they are subjected by the other tissues, and 

 this must have much to do with determining their ultimate 

 size and form. 



The effect of the transverse tension due to the secondary 

 growth in thickness of stems and roots manifests itself very 

 clearly and in various ways. For instance, it is characteristic 

 of the cortical tissues of these organs that their cells, as seen 

 in transverse section, are elongated peripherally. This elonga- 

 tion is the expression of the stretching of the cells under the 

 radial pressure exerted upon them by the growing vascular 

 tissue. In some cases this peripheral stretching may be 

 carried so far that the two walls of the cells meet and the 

 cavities are completely obliterated : Strasburger mentions the 

 older sieve-tubes of Pinus as affording instances of this. 

 Another illustration of the effect of this tension in modifying 

 the form of cells is afforded by the difference in size, as seen 

 in transverse section, between the vessels and cells of the 

 wood produced in the spring and those produced in the 

 autumn, the difference to which is due the marking out of the 

 wood into the " annual rings" which is so conspicuous in 

 the wood of dicotyledonous and coniferous shrubs and trees. 

 The difference in size is due to the fact that the transverse 

 tension is greater in the autumn than in the spring. During 

 the winter the cortical tissue becomes dry and cracks, so that 

 when the formation of new wood from the cambium-layer 

 begins in the spring the transverse tension is relatively small 



