21 



within this constricted part before the removal of the cast. But the 

 question is not of importance since the fact remains that the cast 

 had preserved more pith than would have lived normally. The 

 changes which by comparison with normal plants can be seen to 

 have taken place within the cast are the following: The bundles have 

 grown in number of elements, but not appreciably in size. The thick- 

 walled vessels present when the cast was applied averaged not more 

 than 6 to a bundle ; they are now double that number and distorted, 

 the flattening having no regular direction, being now in the radial 

 direction, now in the tangential and again in an oblique direction. 

 A few vessels which have remained thin-walled have been added and 

 they have not been flattened. The phloem has also increased in 

 number of cells but is also distorted. The meristem between the 

 bundles shows no appreciable change except in a slight radial elon- 

 gation of cells. The cortex shows no change except an appreciable 

 thickening of the membranes of the collenchyma groups. Thus in 

 the bundles where there has been evident change, the room for 

 growth has been mostly if not altogether made within the bundles 

 themselves by the flattening of elements. The minute size of the 

 intercellular spaces present in this stage of the normal plant has not 

 here afforded room for growth that many other plants have shown. 



Above the cast at a distance of 15 mra or less the stem shows 

 normal structure. The number of elements in the bundles is no 

 greater than in the part within the cast (normally of course the lower 

 the section is taken the more elements we should find in bundles), 

 but there is no distortion, and more vessels and phloem cells have 

 had their walls thickened. Moreover the cells bounding each bundle 

 inwardly and those bridging the interfascicular spaces show thickening 

 walls. The collenchyma in the cortex has much thicker membranes 

 than within the cast. 



Below the cast the differences noted between the 2 preceding 

 cases are much more strongly emphasized. There the secondary for- 

 mation has proceeded for some time and the mechanical tissue is 

 strongly developed in and between the bundles and in the cortex. 



In the plant of middle growth the pith within the cast was all 

 living, while above and below were good sized cavities. All of the 

 differences noted in the preceding case between the normal and the 

 constricted portions were more strongly marked in this. Either this stem 

 when encased was less developed internally than in the preceding plant, 

 or the small cavity found within the cast in that plant had afforded 

 room for growth : for in this plant the bundles had fewer elements 

 within the cast than in the corresponding position in the preceding. 



