95 
Endodermis in the Stem of Hippuris. 
abut upon an air-space at either end. The relative position of the original 
cells is preserved, as very little growth of the daughter-cells takes place for 
some time subsequent to the division (Fig. 3). In the outer layers the 
original cells may undergo subdivision into two or three, rarely more. The 
walls may be laid down in any direction, which is however determined by the 
position of neighbouring air-canals, since the ends of these walls invariably 
abut upon these passages. Thus at this stage every intercellular space 
is a centre from which a varying number of walls radiate (Figs. 3, 4). With 
the increase in size of the stem which now follows, the cells of the develop- 
Fig. 3. Transverse section showing the innermost layer of periblem after the division into inner 
and outer cells. / 5 , innermost periblem layer. 
ing cortex undergo further subdivision, whilst at the same time the inter- 
cellular spaces enlarge enormously, owing to the rapid growth of these cells 
which surround, and eventually form single chains of cells separating them. 
The development of the system of intercellular spaces just described 
serves to differentiate, very markedly in the young stem, the nodes and 
internodes, since the air-canals are only developed in the latter. 
In the young nodes, on the other hand, cell-division takes place 
to a greater extent than in the internodes, first in connexion with the 
formation of the leaf-rudiments, secondly with the laying down of the pro- 
