Problem of Xeromorphy in Marsh Plants. 833 
same shoot as 8, A. This leaf had emerged from the bud, and so was 
in contact with the outer air. It was a little further expanded than leaf 6 
in Text-fig. 2 . By this time more xylem tracheae have matured, and many 
of the finer veins, also bearing hairs like the main veins, have been differen- 
tiated. Many stomata have appeared, and also intercellular spaces in the 
mesophyll, but the latter are small, and few of the former quite mature- 
The uppermost layer of mesophyll has begun to take on the character 
of palisade tissue (this is most marked at the tops of the loops), and in 
places even a second row of palisade cells has begun to form. Thus at this 
stage, i. e. very soon after the leaf emerges from the bud, and before it 
is fully expanded, all the chief tissues are recognizable. But the leaf has 
not as yet attained either its full size or thickness. 
A still later stage is seen in Text-fig. 8, C, which represents a section 
through the lowest leaflet of the same leaf as 8, B. (N.B. — The leaflets, like 
the leaves, develop and unfold in regular acropetal succession ; see Text-fig. 2, 
leaf 5.) The leaf has now nearly reached its final thickness. The smallest 
veins are present, also the cuticle and two layers of palisade. The stomata 
are more mature and the intercellular spaces larger. 
The ultimate degree of expansion of the cells and tissues, and also the 
question of whether the lamina shall become flat or remain more or less 
crinkled or folded, depends chiefly on the external conditions. 
The general distribution of pubescence on the leaves has been discussed 
earlier in this paper. It is now clear that the earliest hairs appear on the 
main veins, while the leaf is still in the bud. Subsequently, when the leaf is 
emerging, or has just emerged from the bud, the next hairs to be formed are 
those on the finer veins at the leaf margins or between the main veins ; that 
is, on those parts most remote from the main veins, and which are the first 
to be exposed to the air as the leaf unfolds. 1 * Development of the pube- 
scence may now be arrested, as in the partly hairy leaves or leaflets ; or 
it may proceed further until even the finest veins are covered with hairs. In 
many cases hairs are even formed on the ultimate parenchyma ‘ islands 5 
bounded by the finest veinlets. 
To sum up, the main veins of the lamina, together with the hairs they 
bear, reach a high state of development in the bud. On the other hand, 
the hairs on the finer veins, and those which occur over the parenchyma 
‘ islands ’, only appear when the leaf has emerged from the bud ; i. e. when 
some parts of it are already in direct contact with the outer air. This 
is also true of the palisade tissue, the intercellular spaces of the mesophyll, 
the cuticle, and the stomata. 
We are now in a position to discuss : 
1 la the case of the interveinal bands of hairs, it is the upper surface which is most exposed, 
while the hairs are actually developed on the lower. Cf. Text-fig. S, B. 
