ao8 



MORPHOLOGY OF MEMBERS. 



mother-shoot, and at the same time to the direction of gravitation, h'ght, and pressure 

 (the latter in clinging or climbing plants, such as ivy, Jungermannieae, &c.). It is 

 therefore probable that internal and external causes generally cooperate to determine 

 the direction of the longitudinal axis of a member when first formed, as well as of its 

 lateral shoots \ Further development may show new relations to the original axis and 

 to external influences. The horizontal lateral shoots of numerous woody Dicotyledons 

 with alternate leaves in two rows have the principal section 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. 155); the 

 mid-ribs of the folded leaves are always turned out- 

 wards, 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 

 takes place of such a nature that the principal section 

 assumes a vertical position, the prominent mid-ribs of 

 the leaves turn downwards, while their faces turn up- 

 wards ; and thus the lateral shoot of a horizontal mother- 

 shoot acquires the same position as its parent. The 

 fact that the two rows of leaves within the lateral 

 bud arise on the upper and under side, and consequently 

 both in a vertical plane, might be referred to the im- 

 mediate influence of gravitation ; but this view is 

 opposed by the fact, among others, that the position of 

 the terminal bud^ of the hori^iontal mother-shoot is 

 usually from the first different. In Cercis and Cory /us, 

 for example, the terminal bud stands on the under 

 side of the horizontal end of the branch, and its leaves 

 right and left of the vertical principal section of the 

 bud* The position of a terminal bud may be easily 

 represented by turning Fig. 155 so that the parent axis 

 a lies above, the subtending leaf If beneath th€ bud (the apparently terminal bud thus 

 becoming axillary), and the vertical line t becomes horizontal. This difference, which 

 exists from the very first in the position of the lateral and terminal buds in horizontal 

 bilateral mother-shoots, excludes the immediate influence of gravitation ; but by a useful 

 adaptation, even in the bud all the parts are so arranged that by a single twist of the axis 

 during unfolding, they assume those positions which are most favourable for the functions 

 of the leaves, and by which their faces are turned towards the light. In the terminal 

 buds of such shoots this twisting is not necessary. Whether it is gravitation or the 

 influence of light on growth which occasions this torsion of the axis of the bud is not 



FIG. 155.— Lateral bud of a horizontal branch 

 of Cercis canadensis (in December), in vertical 

 transverse section; i — 7 the' consecutive leaves 

 with their pairs of stipules indicated by the same 

 ' numbers. The outer bud-scales have been re- 

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

 "the centre is the growing- point of the bud. i5 the 

 position of the leaf in whose axil the bud grows ; 

 a axis of the mother-shoot; v direction of gravi- 

 tation. 



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, the reasons for 

 which cannot be explained here in detail. 



^ In order to determine this question experimentally, the apparatus which is described in Book 

 III. Chap. 3, Sect. lo, should be used, a drum which holds the plant, and which rotates slowly 

 round a horizontal axis. 



^. 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 abortion of a 

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

 indifferent that the position of the lateral buds is sometimes not quite horizontal, but a little 

 oblique upwards and outwards, as in Coryhis, Celtis, Sec. 



