2IO 
MORPHOLOGY OF MEMBERS. 
anterior sides even in slender-stemmed species, as B. undulata and incarnata, although 
in these cases the leaves are not approximated on the anterior side, but follow exactly 
the divergence ^. It is well to remark at the outset that the leaf-stalks of Begonias are 
moderately heliotropic, while the axes of the shoot are scarcely so. In thick axes 
heliotropism appears to be entirely wanting ; in slender axes {B. undulata and incarnata) 
it is always very slight. Some species with moderately thick stems, as B. Verschaffeltii 
and manicata, grow upright when the light comes from one side ; very thick-stemmed 
species bend without reference to the direction of the light, while slender-stemmed species 
allow their weak branches to hang down without pointing in any one definite direction. 
If attention be now paid to the direction in which the stems bend, the plane of 
curvature is found always to coincide with the principal section of the shoot which 
divides it into two similar halves, each possessing a row of leaves. A definite relation- 
ship is also manifested between the tendency to bend and the relative thickness and 
length of the internodes. If the thickness of the internodes be represented by i, then, 
in the upright stems of Begonia nitens, Mohringi, and stnuata, their respective lengths 
are 9, 3*2, and 2 ; in the slightly curved B. manicata it is i or less; but in procumbent 
and strongly-curved stems as low as 07 {B. hydrocotylifolia), 0*4 {B. pruinata), and 0*2 
{B. ricinifolia). In the slender-stemmed upright species the rows of leaves are 
diametrically opposite to one another ; in the species with slightly-curved thicker stems 
they approach on the anterior side ; in the very thick-stemmed species which are bent 
downwards the insertions of the leaves are placed entirely on the anterior side^ In 
the thick-stemmed species the stem curves downwards with the anterior side concave, 
or lies procumbent ; but in this case it is always the leafless posterior side which 
faces downwards and puts out adventitious roots {B. ricinifolia and macrophylla). In 
species on the other hand which have tall stems and slender internodes, the branches 
hang down, and in this case it is the posterior side which becomes convex and faces 
upwards {B. undulata and incarnata). Or, in other words, if we look at the buds, in 
the slender-stemmed forms all the larger halves of the leaves when first formed turn 
upwards, while in the thick-stemmed forms they turn downwards. The want of sym- 
metry of the leaves is thus, when the position of the bud is inclined, opposed to 
gravitation, and when the stem is upright has no relationship with it. In species with 
short internodes and thick stems only a few lateral shoots are developed, in those 
with slender stems a great many. Such a relation constantly occurs in other cases 
(Cactaceae, Palms, Ferns, and to an extreme extent in Isoetes). The bilateral arrange- 
ment of the lateral branches is connected with that of the parent axis in the following 
way : — in all species the posterior side of the lateral branch, and hence the larger halves 
of the leaves, faces the parent axis ; the principal section of the former in the slender- 
stemmed species is therefore at right angles to that of the latter. In thick-stemmed 
species, where the axillary shoots are approximated in front, the principal section of the 
lateral branch makes an acute angle with that of the parent axis. As development pro- 
gresses, the branches of slender-stemmed species retain nearly their original position ; 
in thick-stemmed species where the anterior and posterior sides differ greatly, the lateral 
branch twists in such a manner that its posterior side faces in the same direction as that 
of the parent axis. 
I have no precise information as to the mode of life of different species of 
Begonia, but suppose that those species in which the anterior and posterior sides are 
diflferentiated, and which do not cling to the ground, may have the power of climbing, like 
the ivy ; although observations which I have had made for this purpose in the botanical 
garden at Würzburg have not yet led to any satisfactory result, partly because the 
plants were already too old, partly because the access of light was possibly too small on 
^ The absolute measures of thickness run almost parallel to the above-named relative ones ; the 
relatively thickest internodes are also usually absolutely the thickest, and these stems show the most 
decided tendency to a horizontal growth. 
