574 



LECTURE XXXIII. 



It is evident from these observations why longitudinal halves or quarters of 

 growing shoots laid in water curve outwards to such an extraordinary extent — 

 a phenomenon which is exhibited particularly well by the flower-scapes of the 

 Dandelion (Taraxacum officinale) where longitudinal strips lying in water form 

 numerous coils of a helix. 



The passive extension of the epidermis, and of the hypodermal strands of coUen- 

 chyma and sclerenchyma which behave similarly, is thus due chiefly to the vigorous 

 growth in length of the parenchymatous tissues. From this extension in the 

 longitudinal direction, however, a simultaneous transverse tension must necessarily 

 arise. If we suppose a caoutchouc tube extended longitudinally, it is at once noticed 

 that it tends to become smaller in diameter at the same time, and the epidermis with 

 its strengthening layers must behave in the same way when it is passively extended 

 longitudinally. If, on the other hand, a solid cylinder of caoutchouc is supposed to 

 be compressed from above downwards, it becomes at the same time thicker 



transversely, and the growing pith 

 or parenchyma generally of a 

 young stem must behave in just 

 the same manner. These con- 

 siderations at once demonstrate 

 that when the epidermis of a grow- 

 ing internode is extended by the 

 pith, and the pith, on the con- 

 trary, compressed transversely by 

 the epidermis, transverse tension 

 must necessarily occur, and it 

 is at the same time clear that 

 this latter must be a necessary 

 consequence of the longitudinal 

 tension. It has already been men- 

 tioned that this transverse tension 

 may be actually recognised. 



If, however, at the conclusion 

 of growth in length of an inters 

 node, subsequent growth in thickness is ushered in by a cambium zone, a further 

 intensification of this transverse tension results. Since the wood arises on the inner 

 side of the cambium, the whole of the cortex surrounding the wood must necessarily be 

 distended outwards. This tangential extension is perceived with the greatest clearness 

 by means of the microscope, in the form of the cortical cells in transverse sections 

 through the cortex of stems of Rkinus, Helianthus, Sec, which are rapidly growing in 

 thickness. Since, however, the cortex and subsequently even the periderm and bark 

 of woody stems are very slightly extensible and highly elastic, it results that a strong 

 opposite pressure of all the cortical layers lying outside the cambium must respond to 

 the outward pressure due to the growth of the wood; strong tension takes place 

 between cortex and wood, and the cambium itself lies in the position of greatest 

 pressure. We shall learn subsequently how this affects the formation of the wood 

 itself; here it need only be remarked that Gregor Kraus has demonstrated this 



Fig. 3S4. — Transverse section of epidermis and cortex of a Sun-flower, 

 rapidly growing in thickness ; the cells have elongated peripherally and then 

 become divided by radial walls. 



