CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. 211 
the anterior side. The facts stated above do not negative the hypothesis that with 
stronger access of light from one side the stems of Begonias may possibly be aphelio- 
tropic. It appears moreover from Martins ('Flora brasiliensis,' fasc. XXVII. p. 394) 
that at least some Begonias cling to rocks and the stems of trees \ 
Sect. 28. Characteristic Forms of Leaves and Shoots. — The peculi- 
arities of thallomes, leaves, axes, and roots which are common to whole classes, 
orders, or families, are the subject of special morphology and systematic botany; 
on the other hand it is the province of physiology to study the special organi- 
sation by which the members of a plant become adapted to perform definite 
functions. There are, however, some pecuUarities of growth which recur in dif- 
ferent divisions of the vegetable kingdom, or present themselves in striking con- 
trast to the ordinary phenomena, and which are for this reason well adapted to 
bring into prominence the value of general morphological ideas. Peculiarities 
of this kind are termed characteristic, and they must be briefly mentioned here, 
chiefly in order to explain some scientific terms which will be used in Book IL 
We may Hmit our remarks to leaves and leaf-bearing shoots, since the forms 
of the thallus will be treated in sufficient detail in the chapter on Thallophytes, 
and those of roots present only slight characteristic diff"erences, to which refe- 
rence has already been made ; the characteristic forms of hairs has already been 
sufficiently alluded to. 
(i) Forms of Leaves. When fully developed, leaves are usually flatly extended 
plates of tissue, the extension being generally in directions right and left perpendi- 
cularly to the median plane or principal section, so that the surface of the leaf lies 
transversely (at right angles or obliquely) to the longitudinal axis of the stem. This 
is generally quite true for the base of flat leaves ; but the upper part of the surface 
of the leaf is sometimes itself extended in the direction of the median plane, so 
that the plane of extension coincides with an axial longitudinal section of the stem, 
as in the genera Ixia, Iris, &c. But sometimes the leaves are not flat, but conical 
or polyhedral ; conical with almost circular transverse section in Characeas and 
Pilularia, polyhedral in some species of Mesembryanthemum and Aloe. 
The outline of leaves is either simple or segmented ; the former is the case 
when no definitely separated parts can be distinguished in the leaf ; a leaf is 
segmented when it consists of pieces of various shapes, which are more or less 
separated from one another. Leaves which are not flat are usually simple, as 
also are those which are flat but small, their length and breadth being inconsider- 
able relatively to the stem, and not exceeding a few millimetres or centimetres in 
absolute measurement. Larger leaves are usually distinctly segmented, and in 
general the degree of segmentation increases with the increase of size ; the small 
simple leaves of Mosses, for instance, may be contrasted with the large segmented 
leaves of Ferns, the small simple leaves of Lycopodiacese and Conifera? with the 
large compound leaves of Cycadese, the small simple leaves of Linaceae with the large 
much-divided leaves of the nearly-aUied Geraniacese, &c. The segmentation of leaves 
usually consists in the distinction of a basal portion which generally remains narrow, 
cylindrical, or prismatic^ and of an upper portion which is flatly extended ; the 
* On hcliotropism see Book III. Chap. 3. Sect. 8. 
P 2 
