99 
and Affinities of Trapella. 
horizontal walls, separating the different members of any tube. 
Each horizontal wall constitutes a single sieve-plate, both 
faces of which are callous, as shown by appropriate re-agents 
(corallin-soda, &c.). The outermost sieve-tubes of the zone 
have a narrower lumen than the other ones, and qre generally 
(in my material, gathered, July) found obliterated by callus- 
plates (c.p., Fig. 56). The inner ones, though also callous, are 
not sufficiently so to be obliterated. 
In longitudinal section the latter show the familiar schlauch- 
kopfe (s.k., Fig. 55), with quantities of mucilaginous droplets 
and granules, in the neighbourhood of the sieve-plate. Very 
narrow companion-cells, with granular protoplasm and small 
spindle-shaped nuclei, are seen, co-extensive with the sieve- 
tube members. Besides companion-cells, other narrow, elon- 
gated, parenchymatous elements are present, distributed 
throughout the zone of phloem. The layer also lying imme- 
diately within the endodermis belongs to the same category. 
It is from this layer — together with the endodermis to a 
subordinate degree — that the adventitious roots take their 
origin. The sieve-tubes 1 vary in width from -oi6-*oo8 mm. ; 
from each sieve-tube member not more than one companion- 
cell is formed. 
The broad cortex is traversed longitudinally by large air- 
spaces. These, in the submerged parts of the stem, are very 
large (as seen in a transverse section of the stem), extending 
from the axial cylinder almost to the periphery ; there is how- 
ever usually an outer zone of much smaller spaces (Fig. 50). In 
the floating regions these spaces are divided up, so that several 
will lie on any radius. The arrangement indicated will be 
sufficiently understood by a reference to Fig. 49. These 
spaces are entirely schizogenetic in their origin. 
At the nodes, at which are inserted the floating leaves, 
a modification of the cortex is met with, of importance 
1 The occurrence of a continuous zone of well-developed sieve-tubes in the 
reduced vascular cylinder of an aquatic is of interest from their supposed absence 
in certain cases; v. I. B. Balfour, on the genus Halophila, p. 19 in Trans. Bot, 
Soc. Edinb. 1877-78. 
H 2 
