knowledge of the A natomy of Plants . 157 
each of these tracheides is developed by the enormous elonga- 
tion of a single cambial cell, which would involve a very 
striking instance of sliding growth ; on the other hand, Kny 
has found, that the so-called tracheides are in reality short 
vessels arising by the fusion of series of cambial cells. I have 
been at some pains to investigate this question, and though 
my work is far from complete I have satisfied myself that 
cell-fusion does actually occur, while at the ends of the fused 
rows of cells and sometimes at other points independent 
growth goes on. The free ends of the immature vessels 
contain abundant protoplasm, when the rest of the tube 
appears empty ; both this fact and the finely tapering, 
sometimes hair-like points of the vesssls show that some 
independent sliding growth must take place. Occasionally 
a lateral branch arises in the same way. The fact that 
growth of this kind takes place was recognized by Kny. The 
occurrence of sliding growth in connection with cell-fusion is 
already familiar in the case of many laticiferous vessels. 
Much has been done lately on the subject of the apical 
meristem. The theory of germinal layers in plants, founded 
on Hanstein’s discovery of plerome, periblem, and dermatogen, 
has not been able to maintain itself, for even in the highest 
plants these layers are by no means always distinct. As 
pointed out by Sachs, the apical meristem is that part of the 
plant which remains in the embryonic condition. Where we 
have either a single apical cell or a group of equivalent initial 
cells, we may regard the protoplasm of the growing point as 
still retaining the properties of the germ-plasm ; where, how- 
ever, distinct histogenetic layers are present at the apex, their 
cells are already in the way to become somatic in so far as 
they are predestined to give .rise to special tissues only. From 
this point of view it seems clear that an apical cell is quali- 
tatively different from the segments which it cuts off, and 
therefore Sachs’ conception of this cell as a mere gap in the 
system of cell-walls in the growing-point cannot, I think, be 
any longer accepted as sufficient. 
On the closely allied subject of the origin of lateral and 
