l88 EXTERNAL CONFORMATION OF PLANTS. 



of the horizontal end of the branch, and its leaves lie right and left of the vertical prin- 

 cipal section of the bud. The position of a terminal bud may be easily represented by 

 turning Fig. 147 so that the mother-axis a lies above, the mother-leaf b beneath the 

 bud (thus the apparently terminal bud becomes an axillary bud), and the direction of the 

 vertical line 'v becomes horizontal. By this difference which exists from the very first in 

 the position of the bud in horizontal bilateral mother-shoots, the immediate influence 

 of gravity, as conjectured above, is excluded; but a useful arrangement is indicated 

 by the fact that even in the bud all the parts are so arranged that by a single twisting 

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

 the functions of the leaves, and by which their inner surfaces are turned towards the 

 light. In the terminal buds of such shoots this twisting is no longer necessary. 

 Whether it is gravity or the influence of light on the growth which occasions during 

 unfolding this torsion of the axis of the bud through 90° may for the time be left in 

 doubt. But the most important result in reference to the starting-point of this obser- 

 vation is that the principal sections of the axillary buds of a bilateral mother-shoot 

 may have positions greatly varying from the horizontal, and that, in consequence, the 

 arrangement of the parts of the bud is originally independent of the direction of gravity ; 

 while, on the other hand, there results a perfectly definite relationship in the arrange- 

 ment of the parts of the bud to the mother-axis. The axillary bud of such a shoot may 

 arise either laterally or on the under or upper side 1 ; all the leaves always turn their 

 projecting mid-ribs outwards, away from the mother-shoot; the principal section of the 

 bud is always placed in such a position that it is at the same time an axial longitudinal 

 section of the mother-shoot. 



We are led to the same conclusion by the study, among numerous other cases, of 

 two or three-year-old seedlings of Thuja and other Cupressineae. The leaves of the 

 primary stem are arranged below in alternate whorls of fours, and consequently in eight 

 longitudinal rows ; higher up the whorls are alternate and of three leaves, and the primary 

 axis itself has thus six rows. The axillary shoots, the number of which is very small in 

 proportion to the leaves, appear, both in the eight-rowed and in the six-rowed region of 

 the stem, to be generally in two rows, so that in reference to its branching the primary stem 

 is bilateral (other positions of the branches occur, however, higher up, especially later). 

 These lateral shoots of the first order begin at once with alternate whorls of two 

 leaves, or a decussate arrangement, and always in such a manner that the first pair 

 stands right and left of the mother-leaf. Every such lateral shoot of the first order now 

 usually forms a strictly bilateral system of branching, which becomes extended into a 

 plane. This extension-plane of the lateral system of shoots is usually horizontal in 

 seedlings of Thuja gigantea, T. Lobbii, &c., and the principal section therefore vertical. 

 But this is not without exception ; lateral systems of shoots are formed here and there 

 and extend in a vertical plane, the principal section of which is therefore horizontal ; 

 and this is sometimes repeated in single lateral shoots of the second order. Conversely 

 I find on a strong seedling of Cupressus Lawsoniana seventeen lateral systems of shoots 

 (standing in two opposite straight rows on the primary stem) which all extend in a vertical 

 plane, while only one lower system of shoots is extended horizontally. These differences 

 in the position of the principal section of lateral systems of shoots are not however 

 brought about by torsions, which would easily be recognised from the phyllotaxis ; they 

 are original, and become permanent. Where a lateral shoot of the first order branches 

 horizontally, the lateral shoots are produced only from the axils of the leaves that stand 

 right and left ; where it branches vertically, only from the axils of the leaves that stand 

 above and below. Now since the principal sections of these lateral systems of branches 

 have positions altogether diflTerent from the horizontal, it is scarcely possible to suppose 



* Axillary shoots are formed on the upper side of the mother-shoot near its base in Cercis, and 

 bear inflorescences. 



