DIRECTIONS OF GROWTH. I 8q 



that gravity for in this case light) has any immediate influence on the origin of the 

 lateral branches of the second order. The vertical position of the principal section of the 

 horizontal branching of the lateral shoots of the first order is much more constant in 

 jiraucaria excelsa ; and here, as horticultural experience shows, we have a remarkably 

 clear example of a phenomenon which may be considered as inherent to lateral arrange- 

 ment ;— lateral shoots of this kind, planted vertically as cuttings, take root and continue 

 to grow vertically ; but always produce, notwithstanding, only two-rowed lateral shoots ; 

 the branch which has once been produced as a lateral shoot does not become changed, 

 when placed vertically, into a many-sided primary stem '. 



In conclusion I may add a few remarks on the different species of the genus Begonia, 

 which show that in very closely allied forms the relationships of lateral arrangement of 

 parts to external influences may be entirely different, while they remain unchanged when 

 the members of the same plant are compared with one another. The leaves of Begonia 

 are placed alternately in two rows ; on thicker stems the two rows of leaves are nearer 

 to one another on one side of the stem, and hence the other side of the stem appears 

 naked ; the shoot is thus not only bilateral, but it has a leaf-bearing anterior and a naked 

 posterior side which are very unlike one another. The blade of the leaves is very un- 

 symmetrical ; one half is much larger than the other. The larger halves of all the leaves 

 face the posterior side of the stem ; and this can be used to distinguish the posterior and 

 anterior sides even in slender-stemmed species, as B. undulata and incarnata, although in 

 these cases the leaves are not nearer to one another on the anterior side, but follow 

 exactly the divergence |. It is well to remark in the outset that the leaf-stalks ofBegonise 

 are moderately heliotropic, while on the contrary the axes of the shoot are scarcely 

 curved by light. 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 B. manicata, grow straight upright when the light 

 comes from one side ; very thick-stemmed species bend in different directions without 

 reference to the direction of the light; slender-stemmed species allow their weak 

 branches to hang down without always pointing in one definite direction. 



If attention be now paid to the tendency of stems to bend in some one direction, 

 the plane of curvature is found always to coincide with the principal section of the shoot 

 which divides it into two similar halves, so that each half possesses one row of leaves. 

 A definite relationship is also manifested between the tendency to bend and the relative 

 thickness and length of the internodes. If the thickness of the internodes is always re- 

 presented by I, then, in the upright stems of Begonia nitens, Mohringi, and sinuata, 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. ricini/o/ia). In the slender-stemmed upright species the 

 rows of leaves stand diametrically opposite to one another; in the slightly-curved thicker 

 species they approach one another 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 concavely, or lies horizont- 

 ally upon the earth ; but in this case it is always the leafless or posterior side which lies 

 below and puts out adventitious roots (e.g. B. ricini/o/ia and ynacrophylla'). 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 lies above {B. undulata 

 and incarnata') ; or, in other words, if we regard the origin of the buds, in the slender- 



^ [This is not without exception ; see Goeppert, Acta Acad. Nat, Cur. 1868, p. 34, t. i. — Ed.] 

 ^ 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. 



