FORMS OF GROWING-POINTS. 



461 



usually the form of a cone arched outwards like a paraboloid, and which appears 

 simply as the end of the shoot-axis ; in rarer cases, as for example in the water- 

 plants Uirimlaria and Azolla, which are so remarkable also in other respects, 

 this cone is considerably elongated and rolled inwards spirally at its anterior end. 

 When the shoot-axis elongates more slowly, on the contrary, and especially when 

 a flower or an infloresence is to arise at the apex of the shoot concerned, the 

 growing-point generally assumes the form of a flat and broad elevation, which 

 very often becomes eventually extended horizontally, and forms a disc several milli- 

 metres in diameter. This is well seen at the apex of those leaf-shoots of the 

 Compositse (e.g. of the Dahlia and the Sunflower) which are preparing for the 

 formation of flowering capitula : the growing-point, previously conical, is flattened to 

 an almost level disc, on which the embryonic rudiments of young flowers bud 

 forth on all sides from the circumference towards the centre — i.e. the apex of the 

 growing-point. If, on the other hand, 

 a long flowering axis is to be produced, 

 to be covered later with numerous small 

 flowers, as in the Palms, Aroids, and 

 Grasses, the growing-point becomes 

 elongated into a^ cylinder, often several 

 millimetres^ long, on which the lateral 

 shoots of the future panicle, or even 

 the flowers themselves, then arise pro- 

 gressively from below upwards. The 

 subsequent form of the whole inflores- 

 ence is thus already marked out in the 

 form of the growing-point; of course 

 the formative processes connected with 

 the elongation also co-operate later to 

 an important extent, in the completion 

 of the form. 



Even where individual flowers are 

 to be developed, either as lateral out- 

 growths from a growing-point or at the end of leaf-shoots, the flower assumes in its 

 embryonic condition, long before the organs of the flower themselves bud forth, the 

 form of a hemi-spherical dome or flat disc, or very frequently even that of a hollow 

 cup,, whereby the foundation for the subsequent form of the flower is laid: this 

 may then of course again undergo the most various transformations by means of 

 the changes of form connected with elongation. 



The category of these modifications and departures from the ordinary type also 

 includes the far more striking cases of depressed growing-points. A very simple 

 example is afforded by the broad flat shoots of many Liverworts (Metzgeria, Mar- 

 chantiecB) and particularly clearly by the prothallus of Ferns. As soon as these 

 flat leafless shoots attain a certain breadth, the elongating tissue last produced 

 by the growing-point becomes arched forwards right and left in the form of 

 two lobes, between which a deep depression marks the place where the embryonic 

 tissue of the growing-point lies. Fig. 298 v. In this behaviour of leafless shoots. 



FIG. Z97,— Longitudinal section of the apex of the primary stem 

 oiHetianihus annuus, immediately preceding the development of 

 the flowers, j apex of the broad gromng.point ; b b youngest leaves ; 

 r cortex ; m pith. 



