1908] POND—LATERAL ROOTS 419 
of the central cylinder are under compression there can be no doubt, 
and that this compression provides some of the space occupied by 
the advancing root is also very clear. The collapse of some of the 
cells also provides space, and I am inclined to believe that in those 
two ways alone is the space occupied by the ingrowing root to be 
accounted for. Immediately surrounding the radicle, as seen in 
cross-section of the hypocotyl containing it, is to be found a stratum 
of collapsed cells which are so tightly compressed that one cannot 
count the individual cells. The walls are not corroded, however, 
and there is no evidence of wasting away of the tissue or of resorp- 
tion of the cell walls. In some cells the protoplasm shows a little 
indication of wasting away, but it is too slight to count as a factor 
so far as the progress of the in-growing root is concerned. Beyond 
the cells which cannot be distinguished as separate units are those 
Which can be so distinguished but which show compression without 
collapse. I have not in any case been able to account for each cell, 
but more than one-half the number can be found, which, together 
With those too much compressed to be distinguished, plus the space 
Provided by compression, practically accounts for all the space occu- 
pied by the in-grown root. The compression is quite strong, as one 
May find cases in which the central cylinder is widely ruptured with 
the fissure extending for some distance into the cortex. Examination 
Of the cells in immediate contact with the apex of the in-grown root 
Shows them to be intact, so far as any corrosion or wasting-away of 
the walls is concerned. 
Substitution of glass rod for the entering root 
For the sake of greater certainty, preparations like the above were 
made except that a glass rod was vertically pushed into the central 
cylinder by a weight of from 300 to 4008. The rod was drawn out 
'o resemble a root in form. After 48 hours under the weight the rod 
Was removed and the tissues examined. The progress of the rod 
wto the cylinder was somewhat slower than that of the growing root. 
€ examination showed, however, no essential difference in the 
elect upon the tissues of the hypocotyl. One could not say from the 
microscopical examination whether the cylinder had been traversed 
by a root or by a glass rod. The compressed and collapsed cells had 
the Same appearance as seen in other tests with root. ; 
