2ro 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 {E, 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 (J5. 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 

 differentiated, 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 Wiirzburg 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. 



