106 



STKUCTUKAL BOTANY. 



[90, 91. 



two fifths of a circle, or 144. It also shows that in 

 following the spiral from any particular leaf to one 

 directly above it, you must go round the stem twice 

 and pass to the fifth leaf above, and that there are 

 five orthostichies or vertical rows of leaves (Fig. 297). 



299 



299, Phyllotaxy of the cone (cycle 5 \) of Finns serotina. The scales are numbered (1, 2, 3, etc.) in 

 order as they occur in the formative cycle. Between 1 and 22 are 8 turns and 21 scales, etc. 300, 

 Cherry cycle (|) as viewed from above, forming necessarily that kind of aestivation called quincuncial. 



270. It is now known that the angle of divergence 

 varies in different regions of the same shoot ; and 

 that frequently a shoot beginning with a simple ar- 

 rangement, afterward passes on to a more compli- 

 cated pattern. 



CHAPTER XX. 



MORPHOLOGY OF THE LEAF. 



271. The leaf constitutes the verdure of plants, and 

 is by far the most conspicuous and beautiful object in 

 the scenery of nature. It is also of the highest im- 

 portance in the vegetable economy, being the organ 

 of digestion and respiration. It is characterized by a 



