1/4 



EXTERNAL CONFORMATION OF PLANTS. 



The considerations of these constructions leads to definite geometrical rules, by 

 means of which the genetic spiral can be easily deduced from the parastichies \ 

 It is evident that the constructions hitherto mentioned can only be more or 

 less convenient aids to an understanding of the actual principles of the arrangement 

 of leaves. But in order to obtain, with their assistance, a deeper insight into the 

 processes of grow^th themselves of which these principles are the result, it is 

 necessary to follow up the history of development, and in every single case to 

 ask the question, what circumstances are the cause of a new member being formed 

 just in this place and nowhere else. It may be well, therefore, to bring forward 

 here some of the points which must be considered in reference to this view. 



(r) The first question to consider is always the permanence of the order of 



succession which may occur at the time 

 of the origin of the lateral members. 



(2) Attention must be paid not only 

 to the lateral deviation or divergence, but 

 also to the longitudinal distance at which 

 a new member is formed at the punctum 

 vegetationis above the members next pre- 

 ceding it. The longitudinal distances of 

 the youngest lateral structures of a punc- 

 iiim vegetationis from one another are 

 usually very small ; there is often no va- 

 cant space to be distinguished between 

 them ; the planes of insertion of the 

 youngest members are in contact. This 

 circumstance may, on the one hand, 

 assist in the determination of the place 

 where the next member must be produced; 

 but on the other hand may give occasion, 

 as the development of the axis proceeds 

 with its crowded lateral members, to com- 

 pression and distortion, by which the ori- 

 ginal arrangement is altered. 



(3) By the increase in length of the 

 common axis, members w'hich were at 

 first closely crowded become placed at a 

 considerable distance from one another; 

 others, in consequence of slower growth, 

 remain closely packed, so that a different 

 distribution occurs in different parts of the 

 stem (as in the leaf-rosettes and flower- 

 stalks of Crassulaceae, Agave, Aloe, &c.). In the same manner the angle of divergence 

 frequently becomes changed from the more rapid growth in thickness of the axial 



Fig. 146. — Diagrammatic representation of the orthostichies 

 of a i phyllotaxis ; A before and B after the torsion of the 

 stem. Each of the orthostichies /, //, /// is indicated by a 

 double line ; the genetic spiral is single ; where it crosses the 

 orthostichy, the leaf-insertions are indicated by fignres. 



^ As the treatment of the subject is only of value to those who are practically concerned with 

 phyllotaxis, I must refer to the detailed description in Hofmeister's Allgem. Morphologie, § 9. 



