Problem of Xeromorphy in Marsh Plants. 831 
given of the order and time of appearance of certain of the tissues of which 
the lamina is composed. 
If a winter bud of Spiraea Ulmavia be examined about November, it 
will be found that the stipular bud-scales are well developed. The most 
advanced foliage leaves (the glabrous ones of the following spring), however, 
have only reached a stage approximately equivalent to that shown in 
Text-fig. 7 (this figure is taken from a hairy leaf). The leaves remain 
practically in this condition throughout the winter, and are little further 
developed by the following March, when the bud begins to expand. Multi- 
cellular mucilage hairs are abundant, especially on the upper surfaces of the 
veins. But the long cutinized hairs 1 which form the characteristic pube- 
scence of the later leaves are at this stage absent from the entire bud. It is 
Text-fig. 7. Section through part ot a young developing leaf, while in the bud. The still 
meristematic portions (i. e. the interveinal parts of lamina) are shaded, x 360. 
important to notice that few^ if any , of these cutinized hairs are developed in 
the bud until after the first foliage leaves have expanded and become func- 
tional. 
The development of one of the hairy leaves may now be followed. In 
every case the larger veins are the first parts of the lamina to develop, and 
are relatively large and well differentiated at a time when the rest of the 
lamina is still meristematic (see Text-fig. 7). On a leaf such as this, which 
is destined to become hairy, the first hairs are formed early. They appear 
1 At least three kinds of hairs occur on the leaves of Spiraea Ulmaria : (1) capitate mucilage 
hairs, which appear to function mainly in the bud (in Spiraea they wither soon after the leaf expands) ; 
(2) short bristly hairs found scattered on the upper surface and at the margins ; and (3) the long, 
unicellular, twisted, cutinized hairs, of which the pubescence clothing the lower surface of the leaves 
is composed. It is the last kind which is to be understood when ‘hairs 1 are referred to in the 
remainder of this paper. 
