

_ NATURAL ORDERS 131 
or furrows, often of considerable depth, as in the Oak. It 
is on fissured bark that lichens and mosses mostly grow. 
For the branching of Trees see “‘ Klementary Botany,” p. 24. 
In Britain the majority of trees have deciduous leaves ; 
this fact has an important influence on the undergrowth of 
woods, for some leaves decay much more rapidly than others, 
and a richer humus is thus formed. It is said that the best 
“leaf soil” consists of decaying Oak-leaves, and the under- 
growth of the Oak is, as we have already seen, richer than 
that of other trees. There is a close connection between the 
texture of the leaf and its duration. Thin, herbaceous leaves 
last only one season, and drop off in the autumn ; leaves of 
firmer texture last through the winter, only fikus off when 
new leaves are developed, as in the Privet, or they may even 
last for years, as in the Holly. The mechanism producing 
the fall of the leaf is interesting. In some cases the leaves 
merely wither and drop off; in others a layer of cork cells 
several rows deep is formed at the point of insertion of the 
leaf ; the middle row of this dries up, the leaf falls off, and 
the scar of the cork is left on the stem. 
The Amentacez include the following Orders : 
1. Betulacee, comprising the two 8 Birch (Betula) and 
pa (Alas 
2. Cupulifere, comprising the Hornbeam, Hazel, Beech, Oak, 
ee Chestnut. 
3. Salicinee, comprising the Willow and Poplar. 
These resemble each other in having deciduous leaves, 
catkin inflorescences, which in the case of all except the 
Willow and Poplar are borne on the same tree, though on 
different branches ; the flowers are without petals, and the 
fruits, except in the Salicinee, are nuts, which in the 
Cupulifere are situated in cups, called cupules. : 
The The Common Birch is easily recognised by its 
Betulacez. hark, which is silvery, giving the tree its charac- 
teristic appearance ; it is scale bark, and peels off in layers. 
The branches are slender and peculiarly graceful; the sta- 
minate catkins hang down, whilst the pistillate ones of the 
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