THE SPIRAL THEORy. 



50T 



axis t that on the contrary, according to circumstances in each case, special causes 

 determine whether the relations of position turn out to be this or that. Among these 

 causes the most important is the radial or dorsi-ventral character of the growing-point, 

 which in its turn also may again depend upon whether the growing-point in question has 

 arisen as a lateral offshoot of another grow- 

 ing-point. Further, the more or less close 

 crowding of the young organs at the grow- 

 ing-point causes new organs to emerge at 

 definite places, and during the growth dis- 

 placements, torsions, pressures and strains 

 take place. In all this, however, there 

 is still much that remains unexplained — 

 for instance, that the orthotropic radial 

 shoots of the Grasses have their leaves 

 alternating in two rows, while those of the 

 Labiatae have them in decussate pairs; 

 why the radial shoot bears in one case 

 whorls, and in another spirally-arranged 

 leaves ; why vegetative lateral shoots of the 

 Monocotyledons mostly begin with one 

 first leaf turned towards the parent-axis, 

 and lateral shoots of the Dicotyledons 

 (Fig. 337) mostly with two, situated right 



and left ; why in the true Mosses one leaf proceeds from each segment of the two- or 

 three-sided apical cell, and so forth. But the chief point is that we feel ourselves 

 free from the formalism of the doctrine of phyllotaxis, and that these phenomena of 

 growth have now become transferable to the region of causal investigation — i. e. to 

 the physiology of growth. 



Fig. 337. — A young lateral shoot of Spiraa sorbifelia. 

 a the axis of the mother-shoot ; b a leaf developed from this. 

 V growing-point of the young axillary shoot, from which the 

 very young leaf-rudiment i is arising, rtf the first leaves, and 

 e — i the succeeding ones of the young shoot (somewhat highly 

 magnilied}. 



