444 MESSRS. J. PARKIN AND H. H. W. PEARSON ON THE 
The stomata are in some cases sunk, but no very remarkable 
examples have been noticed. Sunken stomata occur in fifteen 
of these plants. On the other hand, twenty-seven have them 
slightly raised or projecting, while the remaining thirty-seven 
- have them about level with the surface of the leaf. The best 
examples of sunken stomata are those of Bupleurum mucronatum 
(Pl. 11. fig. 1), Heptapleurum stellatum (P). 11. figs. 8 & 9), and 
Hypericum mysorense (Pl. 11. fig. 3). Cases of raised stomata 
are shown in figures 3 (Pl. 12) and 6 (Pl. 11) for Atylosia rugosa 
and Didymocarpus Humboldtiana respectively. The sinking of 
the stomata is usually associated with a glabrous surface, and 
the raising with a hairy one; there are a few exceptions in both 
cases. Crotalaria rubiginosa, Gynura Pseudo-china, Lasiosiphon 
eriocephalus, Litsea zeylanica, and Tephrosia tinctoria have a 
hairy undersurface and stomata slightly sunk, while Dodonea 
viscosa *, Hedyotis verticillaris, Senecio zeylanicus, Sopubia 
trifida, Striga lutea, and Viola Patrinii are glabrous and have 
stomata somewhat raised. 
(8) Mesophyli. 
There is a considerable tendency to depart from the typical 
dorsiventral structure, which is distinctly marked in only 
25 per cent of the plants examined. The upper mesophyll 
often passes into the lower by gradations without a distinct 
line separating the palisade from the looser spongy elements. 
In several cases the structure is almost homogeneous, with the 
cells only slightly elongated at right angles to the surface. 
This type of mesophyll, here referred to as isobilateral, 
is usually associated with an erect position of the leaf. That 
form of isobilateralism with a well differentiated row of palisade- 
cells beneath each epidermis hardly occurs. Evolvulus alsinoides 
(Pl. 11. fig. 5), Hypericum mysorense (Pl. 11. fig. 3), Striga lutea, 
and Tephrosia tinctoria—all with erect leaves—have a feeble 
lower palisade-layer. 
The average thickness of the mesophyll for the whole group of 
plants is about 230 yp. Blumea crinita and Gynura Pseudo-china 
are noticeable in having the thickest leaves, the depth of the 
mesophyll in both cases being over 600 p. It is in this respect 
that the leaf of Bluwmea crinita principally differs from that 
of B. flexuosa. 
A double palisade-layer is not very common ; examples where 
* The leaf of this plant is viscid and covered with minute seales. 
