BOTANY OF THE CEYLON PATANAS. 443 
as favouring water-storage by permitting a greater expansion of 
the cell. 
(7) Stomata. 
The occurrence of stomata on the upper surface is fairly 
common. Thirty-two plants have them thus situated. In six 
of these they are very scarce, less than one persq.mm. In 
eleven they are fairly numerous, but only half as many, or 
less than on the lower surface. In fifteen they are equally 
distributed, or nearly so, on both surfaces, and in these with one 
exception, Layenophora Billardieri, the leaf is either erect ur 
semi-erect, or shows sun-movements. In one case, Zornia 
diphylla, a papilionaceous plant with leaf-movements, the 
number on the upper surface is actually larger than on the 
lower. Anaphalis oblonga, Gynura Pseudo-china, Lagenophora 
Billardieri, Microglossa zeylanica, Senecio zeylanicus, and Viola 
Patrinii, the leaves of which have stomata fairly numerous on 
the upper surface and yet are not erect nor possessed of move- 
-ments, do not show xerophytic characters, at any rate in their 
mesophyll; they may be considered mesophytes and all, save 
one, are Composites. 
The number of stomata per square millimetre varies greatly 
for different species. Practically, nothing is known of the 
conditions which govern this variation. It is perhaps not so 
much their number as the size of the aperture which is to be 
taken into account. The average number per sq. mm. of the 
lower surface for this group of plants is 284, a figure some- 
what larger than an average given by Weiss* for a miscellaneous 
collection of plants. Psidium Guyava is very remarkable 
in having as many as 1000 per sq. mm. Two species of 
Eugenia and Rhodomyrtus tomentosa belonging to the same 
natural order, also Jasminum angustifolium, Ligustrum Walkeri, 
Rhododendron arboreum, Actinodaphne stenophylla, and Atylosia 
Candollei, have large numbers. If both surfaces are taken into 
account together, which is perhaps the legitimate plan, then 
Cassia mimosoides stands next to Psidium Guyava as having the 
second largest number, viz. 800; Hypericum mysorense follows 
close with 750. Bupleurum mucronatum, Cassia Kleinit, Crota- 
laria albida, Osyris arborea, and Striga lutea have numbers 
above 500. 
* Weiss, Anat. der Pflanzen (1878), p- 392. 
