496 



LECTURE XXIX. 



Fig. 332.— Transverse ; 



L of a shoot oi Aloe j 



numerous cases of decussate pairs of leaves, as in the Labiatas, contradict in every 

 way even the merely formal arrangement in a so-called genetic spiral running round 

 the shoot-axis. This latter, moreover, even where it can be carried out, viz. in the 

 case of radial shoots with scattered leaves, is only subjective and imagined as 

 belonging to the plant by the observer, and has no significance whatever for the 

 knowledge of the processes of growth themselves. Innumerable cases may be 

 adduced from which it results definitely that the arrangement of the leaves and 

 lateral shoots on a shoot-axis essentially depends upon whether the latter is already 



radially or dorsi-ventrally constructed 

 at its growing-point. A few exam- 

 ples only will serve to illustrate this. 

 Fig. 332 represents a transverse sec- 

 tion through the lateral shoot of an 

 Aloe. It grew out from the mother- 

 shoot at first horizontally, and pro- 

 duced its leaves right and left in two 

 alternating rows, as may be seen by 

 the numbers 1-6 : eventually the 

 growing-point of this shoot took an upward direction, and it became orthotropic 

 and radial. This change found its expression in the fact that the primitively straight 

 rows of leaves now pass over into two spirally-wound ones, as is to be seen from the 

 two dotted lines 7, 9, 11, 13, 15, and 8, 10, 12, 14. The completely erect slioot 

 bears a rosette of leaves radiating on all sides. 



Among the inflorescences of Phanerogams are to be found very many, especially 

 in the families of the Boragineae, Papilionaceae, and Urticacese, which are termed 

 unilateral in descriptive botany, that is to say, the shoot-axis from which the more or 

 less numerous flowers arise as lateral shoots is itself 

 dorsi- ventral, and therefore produces flower-shoots only 

 on its dorsal or only on its ventral side. The spiral 

 theory was compelled, in order to vindicate itself 

 in such cases, to put forward the most extraordinary 

 and improbable accessory hypotheses. By means of 

 careful investigation of the processes at the growing- 

 point, Goebel showed, however, that it is altogether 

 impossible to entertain the spiral theory in such cases, 

 simply because the leaves on the dorsi-ventral parent 

 axis bud forth, as a matter of fact, only on one side, in 

 one, two, or more rows. Fig. 333 shows this behaviour 

 at the growing-point of an inflorescence oi Symphytum, 

 and the case is exactly the same in the common Forget-me-not {Myosotis). In the 

 latter plant especially, any one may easily perceive how the axis of the inflorescence, 

 which is rolled spirally inwards and downwards, bears flowers arranged in two rows 

 on its convex upper side only : the mature state observed with the unaided eye, 

 as well as the processes at the growing-point, forbid every attempt to speak here of 

 a spiral arrangement. 



But even in individual flowers the case occurs that the diff"erent phylloid organs, 



Fig. 333.— Young inflorescence 

 of Symphytum, v growing-point ; 

 * b the young flowers. 



