26 



PART I. THE MORPHOLOGY OF PLANTS. 



[4. 



FIG. 13. Diagi-am of mul- 

 tilateral scattered arrange- 

 ment, with divergence of . 



common axis, their divergence is constant; that is, that the distance 

 between any member and its immediate predecessor and successor is 

 a certain fraction of the circumference. In a simple case, when the 

 divergence is -^ (Fig. 13), starting with any 

 member 0, the insertion of the next mem- 

 ber developed in acropetal succession on 

 the common axis, which may be numbered 

 1, will be separated from that of by just 

 |- of the circumference, and the next mem- 

 ber, numbered 2, will be separated from 1 

 by I of the circumference, and 3 from 2, 

 and so on. Hence 3 lies directly over 0, 

 4 over 1, 5 over 2, and so on ; so that there 

 are three orthostichies. In proceeding from 

 to 1, 2, 3, and so on, always in the same 

 direction, the circumference of the common axis is traversed in a 

 spiral which, in the course of each whole turn, touches the bases 

 of three lateral members and intersects the same orthostichy. 

 This spiral will pass through the insertion of every lateral mem- 

 ber, and as it does so in the order of their development, it is 

 known as the genetic spiral. The number of lateral members 

 through which the genetic spiral passes in its course between any 



two on the same ortho- 

 stichy, is termed a cycle. 



It might, however, be 

 said with equal accuracy, 

 that the divergence is ^, 

 and that by passing from 

 member to member by | 

 of the circumference, a 

 spiral would be traced 

 which connects the mem- 

 bers in genetic sequence. 

 But in this case two turns 

 of the spiral would have 

 to be traversed before re- 

 turning to the orthostichy 

 started from ; i.e. the 

 cycle will consist of two 



turns of the spiral instead of one. For the sake of simplicity, the 

 spiral is not traced in this longer way, but in the shorter wny. 



FIG. 14. Diagram of a shoot with a constant 

 divergence of |. I, II, III, etc., the orthostichous 

 lines. (Alter Sachs.) 



