Problem of Xeromorphy in Marsh Plants . 827 
stomata on the upper leaves extraordinarily high, 1 but the range from about 
300 to nearly 1,300 per sq. mm. for different leaves of the same shoot is 
equally remarkable. It maybe that Spiraea Ulmaria is exceptional in the 
latter respect. But it would be worth while comparing the successive leaves 
of a number of other plants, for if the phenomenon is at all general, the 
numbers already published for a large number of species 2 might need con- 
siderable revision. The highest number previously recorded appears to be 
1,000 per sq. mm. in Psidium Guyava , which Parkin and Pearson 3 mention 
as being very remarkable in this respect. 
The glabrous autumn leaves resemble the glabrous spring leaves in 
structure as well as appearance. Thus the number of stomata in the former 
is usually between aoo and 300 per sq. mm. Further, in the case of tall 
radical leaves, there is often considerable difference between the upper and 
lower leaflets, the latter possessing a more pronounced ‘ shade ’ structure 
than the former. 4 Thus the lower leaflets may be less hairy, and have 
fewer stomata and more lacunar 
mesophyll. E. g. the sixth leaf 
of Shoot A had 235 stomata per 
sq. mm. on the terminal leaflet, 
and only 213 on the lowest leaflet. 
Again, in a partly hairy leaf (the 
fourth on the shoot) collected at 
Wicken Fen, the terminal leaflet 
had 395 stomata per sq. mm. ; the 
second pair of leaflets, 333 ; the fourth pair,. 269, and the lowest pair of 
all, only 176. In other cases, however, the differences were less than these. 
But the number of stomata is not the only thing to be considered. 
From the point of view of transpiration, the size of the stomata, and more 
particularly of their pores, is as important as their number. Text-fig. 6 
shows stomata taken from three different leaves. A is from a radical leaf of 
a plant grown in deep shade, under a bell-glass, i. e. in saturated air and 
weak illumination. The pore is widely open. The leaf had an average of 
255 stomata per sq. mm. B is from the second leaf of Shoot C, with 303 
stomata per sq. mm. Here the pore is smaller, but still much larger than 
that of C, which is taken from the eighteenth leaf of the same shoot (C), 
1 Shoot C was not unique in possessing such numerous stomata. In all, three shoots of var. 
denudata were examined, and in each case the stomata on the upper leaves exceeded x,ooo per sq. mm. 
In one shoot 1,252 were recorded, and in another, grown in shade, but otherwise exposed, 1,112. 
2 Cf. Morren (’63), Weiss (’65), and other authors. Miss Delf (’ll), p. 500, has recently recorded 
a similar rise (from 97*5 to 236.25 per sq. mm.) in the number of stomata on successive internodes of 
Salicornia annua. 
3 Parkin and Pearson (’03), p. 443. Jost (’07), p. 38, states that in extreme cases there may be 
as many as 625 ( Olea ) or 716 ( Brassica rapa) per sq. mm. 
4 Boodle (’04), pp. 663-4, records an instance in which the upper, exposed pinnae of a leaf of 
Pteris aquilina had marked xerophytic characters, which were absent in the lower, sheltered pinnae. 
