156 The Botanical Gazette. [April, 
pith entirely thin-walled, when above and below the casts the 
zone of fully thickened pith-cells has been six cells in radial 
width. 
If we turn now to woody plants we shall find the same re- 
sults presenting themselves. Several shoots of Melianthus 
major had casts placed around them so as to leave only the 
terminal bud exposed above. Up to the time when three in- 
ternodes had been subsequently developed there were no 
thick-walled cells in the pith of the segment in cast, but above 
and below the cast there was a broad band of thick-walled 
lignified pith. In shoots similarly prepared but of farther 
development above the casts, there was always an evident 
thickening of the outer pith-cells within the limits of the casts, 
this thickening progressing in the older preparations till it 
approached that of like cells outside the confined area. The 
same general results were obtained by similar experiments 
with Forsythia viridissima and Pterocarya fraxinifolia. 
Many plants which have collenchyma in the cortex of the 
young stem do not, as is well known, increase the amount of 
this tissue as growth proceeds, while others with increasing 
- age in an internode show the collenchyma increasing in num- — 
ber of cells. and thickness of membrane. Sambucus nigra be- 
longs to the latter class. When in the spring very youns 
shoots have some of their internodes enclosed in gypsum and 
are allowed to grow subsequently, the increase of the col- 
lenchyma is found to be more tardy within the casts than 
outside of them, though the thickening of cell-walls 's stil 
In the young stems of Archangelica sativa and My se 
a 
these cells would have remained thin-walled within the ¢ re 
was not determined by other experiments; but that they is 
still capable of growth there can be no doubt, for they-97 
at the time of examination well provided with protoplasm. 
The fact that the sclerenchyma of the fundamen 
develops more slowly against mechanical resistance has 
tal tissue 
