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86 Structural and Physiological Botany. : 
on the contrary are zzvolute (Fig. 136), or rolled inwards — 
- ‘towards the mid-rib. The petals of Convolvulus (Fig. 137) 
are contorted spirally ; those of the poppy irregularly crampled. 
With respect to their anatomical structure, it may be re- 
marked that buds are especially distinguished by the pre- 
sence of formative tissues. 
THE LEAVES. 
It will be seen from what has now been said that the 
Leaves are appendicular organs of the stem. ‘The apex is 
always formed first, and is therefore the oldest part of the * 
leaf; the remaining parts being gradually interposed between 
it and the stem. The order of development is therefore the 
reverse in the leaf to what it is in the stem, where the apices 
are the youngest parts. 
The various positions and functions of leaves neces 
their classification into four kinds :—seed-leaves or cotyledons, — 
covering-leaves, foliage-leaves, and floral leaves. ‘The first of 
these kinds will be more fully described when speaking of 
the seed ; the last when speaking of the flower. 
Covering-leaves are so called because they cover or pro- 
tect other parts of the plant. In this class are included the 
bud-scales already described (p. 83), and the dracts or 
hypsophyllary leaves, z.e. those leaves in the axils of which 
the flowers are placed. The bud-scales are mostly brown 
bodies but little developed, which fall off as the develop- 
ment of the bud progresses, when they have fulfilled their 
purpose.’ The bracts are, on the contrary, as a rule more 
perfectly formed and green, or of some other colour as in 
the spathe of Cala [or Richardza, or the brilliant red bracts 
of Pornsetiza, Buginvillea, &c.| Their presence or absence, 
and their more or less complete development, are important in 
determining the nature of the inflorescence, and in defining _ 
the characters of certain families of plants. The g/umes of 
Grasses, the spathe of Aroidez, the alee of Composite, and — 
