388 LECTURE XVI. 



Transverse section. Normal. Etiolated. 



Radial diameter of an entire vascular bundle 95 55 



the hard bast 16 8 



soft (incl. cambium) 9 12 



xylem 55 22 



medullary sheath 20 13 



medullary cells 15-2 51 



Longitudinal section. 



Mean length of the epidermal cells 5-5 13-5 



cortical 12*2 32-4 



hard bast fibres 90 89 



wood 88 88 



medullary cells 29*5 62 



The conclusion arrived at by observations of this kind 

 by Kraus and by Rauwenhoff is this, that the excessive 

 elongation of etiolated internodes is due to an exaggerated__ 

 growth of the parenchymatous cells, a conclusion which the 

 facts seem fully to justify. But this is not a final explanation, 

 for the exaggerated growth of the parenchymatous cells has 

 yet to be accounted for. 



II Kraus attributes this exaggerated growth principally to 

 Ikhe thinness of the walls of the epidermal, collenchymatous, 

 rand sclerenchymatous cells, in consequence of which they 

 / remain extensible and offer but little resistance to the elonga- 

 tion of the parenchymatous cells. He considers that an 

 etiolated internode remains in that condition which, as men- 

 tioned in a previous lecture (p. 346), is characteristic of very 

 young normal internodes. In such an internode, it will be 

 remembered, the parenchymatous tissue (pith) does not 

 elongate on isolation, but the other tissues shorten. If this be 

 so, then, in the etiolated as in the young normal internode, 

 the longitudinal tissue-tension must be small, and Kraus 

 adduces observations which shew that this is, sometimes at 

 least, actually the case. Again, if this be so, then internodes 

 which, when grown under normal conditions, exhibit longitu- 

 dinal tissue-tension in a high degree, should be especially 

 remarkable for excessive elongation when grown in continuous 

 darkness. Kraus mentions the long narrow leaves of various 

 Monocotyledons, such as the Onion, the Hyacinth, the Crocus, 



