46 



has been in most eases a crushing of primary vessels. In the space 

 thus made the meristem during the first few weeks slowly produces 

 new cells and the more vigorous of the immature cells gradually 

 expand. Usually with this slight growth a few cells begin to show 

 thickening walls. If the pith should not be resistant, as it is not in 

 Cucurbita, or if the cortex should be crushed, as in Ricinus, farther 

 growth will take place and there will be more thickening of cell- 

 walls. But even under such conditions if the pressure is great the 

 amount of mechanical tissue will be far below normal. 



Several plants that have reached a large growth after the cast 

 was applied and thus have made a great transpiring surface above the 

 position of the cast have shown a very slow but gradual thickening 

 of xylem cells without showing an increase in size of such cells. 

 Such plants are Vicia and Ricinus. It is possible in these cases that 

 transpiration has been the stimulus for such growth. If as Kohl's 1 

 experiments seem to show the amount of thick-walled tissue is in- 

 creased or decreased with the increase or decrease of transpiration, 

 we should expect to find a corresponding regulation in such a nar- 

 row neck of stem as that within the cast where the cross-section is 

 often no more than one-fourth that of the same stem in normal posi- 

 tions. Yet such an effect by transpiration is not great enough in 

 these experiments to allow at present a positive statement to be made 

 regarding it. 



It must not be supposed that the enforced small size of cells is 

 the one sufficient condition for the thinness of wall. As mentioned 

 before, the behaviour of tissues within the casts, but within a centi- 

 meter of the limits, is quite different from what has just been de- 

 scribed for the middle of the cast. In those stems where the casts 

 have been applied before primary growth has ended and where 

 such stems have made a fair subsequent growth , the mechanical 

 tissue in the neighborhood of the limits of the casts has always been 

 abnormally great. Just outside the limits of the cast where the stem 

 has nearly reached its normal thickness the amount of such thick- 

 walled tissue is greater than anywhere else in the stem. Just within 

 the limits of the cast the amount of such tissue is greater than 

 elsewhere in the cast and greater than occurs in tissues of equal 

 development in normal parts of the stem. As an illustration to make 

 the meaning of the last more clear, it may be said that in the 

 middle of the part of the stem within the cast the xylem cells 

 among the vessels and the pith-cells bounding the bundles will be 



1 Kohl, Die Transpiration der Pflanzen. Braunschweig 1886. 



