DIRECTIONS OF GROWTH. 209 



the question now under discussion. But the most important result is that the principal 

 sections of the axillary buds of a bilateral mother-shoot may have greatly varying posi- 

 tions, and that, in consequence, the arrangement of the parts of the bud is independent 

 of gravitation ; on the other hand, there is a definite relationship to the mother-axis 

 in the arrangement of the parts of the bud. The axillary bud of such a shoot may 

 arise either laterally or on the under or upper side ^ ; in all cases its leaves turn their 

 prominent mid-ribs outwards, away from the mother-shoot, so that the principal section 

 of the bud is always at the same time an axial longitudinal section of the mother-shoot. 



We are led to the same conclusion by the study of two- or three-year-old seed- 

 lings of Thuja and other Gupressineae. The leaves of the primary stem are arranged 

 below in alternating whorls of fours, and consequently in eight longitudinal rows; 

 higher up the whorls are alternate and of three leaves, and the leaves are in 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 alternating whorls of two leaves, or a 

 decussate arrangement; and the first pair always stands right and left of the subtend- 

 ing-leaf. Every such lateral shoot of the first order usually produces a bilateral 

 branch-system, which spreads out in one plane. This plane is usually horizontal in 

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

 But this is not without exception; lateral branch-systems are formed here and there 

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

 this is sometimes repeated in some of the lateral shoots of the second order. I found, 

 however, on a strong seedling of Cupressus Lanusoniana seventeen lateral systems of 

 shoots (standing in two opposite rows on the primary stem) all spreading in a vertical 

 plane, while only one lower system spread horizontally. These differences in the 

 position of the principal section of lateral branch-systems are not however brought 

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

 original, and permanent. Where a lateral shoot of the first order branches hori- 

 zontally, its branches are produced only from the axils of leaves that stand right and 

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

 below. Now since the principal sections of these lateral branch-systems have altogether 

 different inclinations to the horizon, it is scarcely possible to suppose that gravitation 

 (or light) has any immediate influence on the origin of the lateral branches of the 

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

 first order is much more constant in Araucaria excelsa ; and here, as horticultural 

 experience shows, we have a phenomenon inherent to lateral arrangement; — lateral 

 shoots, planted verticcUly as cuttings, take root and continue to grow , vertically, but 

 produce only lateral shoots in two rows ; the branch which has once been produced as 

 a lateral shoot does not change, when placed vertically, into a many-sided primary stem^. 



I may add a few remarks on the species of the genus Begonia, which show that the 

 relation of the bilateral arrangement of the parts to external influences may be entirely 

 different in very closely allied forms, while it is constant for members of the same plant. 

 The leaves of Begonias are alternate in two rows; on thick stems the two rows 

 approximate on one side of the stem, and 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. The blade of the leaves is very unsymmetrical, 

 one half being much larger. The larger halves of all the leaves are turned towards 

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



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

 bear inflorescences. 



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



P 



