100 
appear between them. An interchange of gases between the ex- 
ternal air and the palisade cells is therefore permitted, but to a 
restricted extent. In addition, the palisade cells tit compactly 
together, so the loss of water vapour must ot necessity be slow. 
The inner region of the leaf contains large parenchymatous 
water-storing cells, but loss of water to the outer border is 
restricted by the interposition of a row of mechanical fibres. The 
portion of the section within these fibres has a structure somewhat 
similar to that of a monocotylcdonous stem. Vascular bundles 
appear throughout the section. On examination it is seen that 
they are not scattered indiscriminately, but appear in more or less 
definite lines seen running laterally across Fig. A of Plate XVII. 
As in Zea mais, each bundle is enclosed in a sheath of mechanical 
tissue. Within this are two distinct sets of xylem cells forming a 
“V” and almost meeting at the apex which is directed towards the 
central parts of the leaf. The outer ends of the xylem sets enclose 
the phloem, which is cleft in two by a median strand of 
sclerenchyma. The bundles nearest the leaf surface are imper- 
fectly formed, being in fact the ends of vascular strands which 
occupy a more central position in a section at a lower level of the 
leaf* Two such bundles are shown in Fig, B, Plate XVII., and in 
them the xylem sets are not well marked. Each bundle is so 
oriented that the phloem is directed towards the nearest surface 
of the leaf. 
The chief xerophytic characters shown in the leaf structure 
of the Blackboy are: — 
(1) Water-storage tissues in central portions of leaf. 
(2) Prevention of excessive transpiration by — 
(a) limiting of stomata to strips of leaf surface over- 
lying the palisade cells; 
(b) a thick cuticle; 
(c) removal of delicate parenchymatous cells from 
epidermal layer by the interposition of much 
selerenchymatous tissue ; 
(d) Minimising water loss through stomata by blunt 
hairs, narrow entrances, and an almost continuous 
layer of woody fibres which separates the stomatal 
cavity from the parenchymatous cells below. 
(3) The water distributing xylem elements of the vascular 
bundles are strongly developed. 
Each leaf examined shows some adaptation for water con- 
servation. and though in many leaves the structural features are 
very similar, it happens that now and again some rarer device is 
displayed. The origin of these features to which the plants owe 
