THE STEM 63 
2. The layer of two rows of parenchymatous cells contain- 
ing chlorophyll, immediately beneath the epidermis. These 
cells resemble those usually found beneath the upper epidermis 
of the leaf; they play an active part in assimilation. 
3. The water-storage tissue. The cells of which this con- 
sists have thin walls and colourless cell-contents. | 
4. The fibro-vascular tissue, consisting of bast, cambium, 
and wood, is 
very much re- 
duced com- 
pared with that 
of a Mesophyte. 
The wood is re- 
duced because 
there is little 
transpiration ; 
therefore con- 
duction of water 
is not so neces- 
sary ; then, the 
assimilating tis- 
sue being pre- 
sent in the stem, 
the bast is re- 
duced, as the 
distance along 
which the food 
is to be con- 
ducted is short. 
Stem of In the 
_Hydro- gtem of 
phytes. : 
aquatic 
ba 
ree 2 
see 
ig me 
$: 
S 
HA $ pitty 
J 
'b: 
* 
* ty hed 
tS geen, 
Sor SII 
2, 
on. 
xb %; ie 
tee 
i 
2%. 
a ee 
6 SFY 
eg 3 
Ke 
ae, 
ro e2 
+ s 

Fic. 32.—Hirruris Srem, TRANSVERSE SECTION. 
(Low power. ) 
e, epidermis ; cort, cortical tissue with large inter- 
cellular spaces ; end, endodermis ; st, stele. 
plants the epidermis has either no cuticle or a very thin one. 
In Mare’s-tail (Hippuris vulgaris) there is a thin cuticle. 
The cortical parenchyma is characterised by very large inter- 
cellular spaces, which give lightness to the plant. The central 
vascular cylinder is very much reduced ; the wood occupies a 
