664, MR. Le Ae BOODLE ON THE STRUCTURE OF LEAVES 
xerophytic characters. In fact, the upper and lower parts have a 
structure identical with that of leaves from very exposed and very 
sheltered habitats respectively, though not quite extreme. It may 
be deduced from this that, not only does the structure depend on 
the environment, but the mature structure is only determined at 
a rather late stage in the growth of the leaf, viz. in this case 
when the leaf had grown taller than the bush, which it did not 
greatly exceed in height *. 
It remains now to describe the cultural experiment. In the 
spring of 1902 a rhizome of the bracken (of intermediate type) 
was dug up and transferred to a greenhouse (heated) attached to 
the Jodrell Laboratory. he greenhouse faces nearly south ; 
the air in it was kept very moist ; the temperature varied a good 
deal, but was not often below 65° I., while, especially during 
bright sunlight in the summer, it often reached 80° and some- 
times 90°. All the leaves produced in the greenhouse were soft 
and very delicate in appearance f, and in structure they closely 
resembled leaves from very dense shade and shelter, palisade not 
being differentiated or only slightly. This fact suggests that in 
the bracken, light (at any rate when not intense) is not the all- 
important factor ¢ determining the structure of sun-leaves and 
shade-leaves. Unfortunately no readings were taken with an 
actinometer ; but the illumination was certainly strong, and must 
have been decidedly stronger than in certain habitats under the 
shade of oak-trees, where the bracken-leaves might be called 
half-tough and have a fair amount of palisade. 
The nature of the experiment renders the results inconclusive, 
but the deduction suggested above gains in probability wher 
taken in connection with Eberhardt’s results §. In each of his 
experiments with flowering-plants three plants were used, all 
* The reverse of this was found by Nordhausen (“ Ueber Sonnen- und Schatten- 
blitter,” Ber. d. deutsch. bot. Gesellsch. 1903, p. 30) to apply to “‘ sun-branches” 
and ‘“shade-branches” of trees and shrubs (e.g. the beech), in which the 
character of the mature leaf isto a great extent determined in the leaf-rudiments 
within the winter-buds. 
t+ There are one or two leaves, in several respects resembling these, in the 
Kew Herbarium. They are named Preris aquilina, L., var., and they come 
from the Tropical Fern House, where they were produced in 1901 by a piece 
of rhizome accidentally introduced, as Mr. C, H. Wright informs me. 
{ As assumed by Stahl, Jenaische Zeitschr. xvi. (1883) p. 182. 
§ Eberhardt, “Infl. de lair sec et de l’air humide sur la forme et la struct. 
des végétaux,” Ann, Sei. Nat. Bot. 8° sér. t. xviii, 1903, p. 61. 
