DIRECTIONS OF GROWTH. 



87 



dichotomy, one side of the shoot chnging closely to the ground or to upright bodies (as 

 in Hepaticae), or one side turns to the light, the other side to the shade {e. g. Selaginella) ; 

 in such cases the shoots are also broader in the direction of the plane of dichotomy. 

 Where no such different development of the two sides occurs, as in Lycopodium (espe- 

 cially L. Selago according to Cramer), the dichotomy of consecutive bifurcations may 

 take place in different planes ; and this is also the case with the roots of Lycopodiaceae 

 (cf. Nageli and Leitgeb, and Pfeffer, I.e. p. 97). 



As has already been mentioned, it is usually impossible, without experimental 

 research, to determine whether the position of the principal section of bilateral shoots 

 and leaves depends immediately on their relation to the mother-shoot, or is directly 

 brought about by external conditions, such as pressure, gravity, and lights The position 

 of the principal section usually shows simultaneously definite relations to the mother- 

 shoot, and to the direction of gravity, light, and pressure (the latter in clinging or 

 climbing plants, as ivy, Jungermanniae, &c.). It is therefore probable that internal and 

 external causes generally cooperate to give a definite direction to the longitudinal axis 

 of a member when first formed, as well as definite positions to its lateral shoots. As 

 development advances, the relative positions may change, and may show new relations 

 to the original axis and to external influences. On this point reference may be made to 

 the horizontal lateral shoots of numerous woody 

 plants among Dicotyledons with alternate leaves in 

 two rows. Their principal section is vertical, their 

 rows of leaves right and left. The axillary buds of 

 these leaves which remain dormant through the winter 

 show an altogether different disposition of their parts; 

 the axis of the bud is parallel to that of the mother- 

 shoot ; it bears its leaves in two rows, one facing 

 the sky and the other the earth (Fig. 147); the mid- 

 ribs of the folded leaves are always turned outwards, 

 away from the mother-axis ; the principal axis of the 

 whole bilateral shoot (the bud) is horizontal. But 

 when the bud unfolds in the spring, a torsion of its 

 axial structure takes place of such a nature that the 

 principal section assumes a vertical position, the 

 projecting mid-ribs of the leaves turn downwards, 

 while the lobes of the lamina turn their sides, 

 which were previously inclined towards one another, 

 upwards ; and thus the lateral shoot of a horizontal 

 mother-shoot acquires the same position as this latter. 

 The fact that the two rows of leaves within the lateral 

 bud arise on the upper and under side, and con- 

 sequently both in the vertical plane, may be referred 

 to the immediate influence of gravity ; but to this 

 is opposed, among others, the fact that the posi- ♦ 



tion of the terminal bud^ of the horizontal mother-shoot is usually from the first dif- 

 ferent. In Ccrcis and Corylus, for example, the terminal bud stands on the under side 



* This subject has been treated by Hofmeister (Allgemeine Morph. §§23, 24) from another 

 point of view ; but on consulting the facts themselves I find much that is not in agreement with his 

 statements, and in their interpretation I arrive at essentially different conclusions which cannot be 

 explained here in detail. 



^ It is for our present purpose the same whether the bud at the end of the horizontal shoot be 

 its true terminal bud, or a lateral bud the development of which is induced by the withering of a 

 terminal bud, as in Cercis and Corylus. In reference to the position of the terminal bud it is also 

 indifferent that the lateral buds sometimes place their principal section not quite horizontally, but a 

 little obliquely upwards and outwards, as in Corylus, Celtis, &c. 



Fig. 147.— Lateral bud of a horizontal branch 

 of Cercis canadensis (in December), in vertical 

 transverse section ; 1-7 the consecutive leaves 

 with their pairs of stipules indicated in the same 

 manner. The outer bud-scales have been re- 

 moved, the two inner ones indicated by 3, 3. In 

 the centre is the fiuvctuni vegetationis of the 

 bud. b the position of the leaf in whose axil the 

 bud grows ; a axis of the mother-shoot ; v di- 

 rection of gravitation. 



