DIV. I 



MORPHOLOGY 



89 



farther out are successively older and lower. It is convenient to 

 indicate each node by a circle ; when there are several leaves at the 

 same node they are represented on the same circle. Such diagrams 

 agree with the figures of transverse sections of a bud in the neighbour- 

 hood of the apex of the stem (Figs. 99, 104). 



It thus appears that EVEN AT THEIR APPEARANCE THE LEAVES ON 



AN ERECT RADIAL SHOOT ARE DISPOSED AS REGULARLY AS POSSIBLE 

 AROUND THE STEM. THIS ENSURES THAT THE EXPANDED LEAVES DO 

 NOT SHADE ONE ANOTHER BUT MAKE THE FULLEST POSSIBLE USE OF 



THE LIGHT. The distribution is so regular that the angle between 

 two successive leaves (e.g. in Fig. 105, leaves 1 and 2, 2 and 3, 

 etc.) is constant ; this is termed the ANGLE OF DIVERGENCE, or, when 



FIG. 104. Transverse section of a leaf- bud 

 of Tsuga canadensis, just below the 

 apex of the shoot, showing a ^ diverg- 

 ence, (x circa 20. After HOFMEISTER.) 



FIG. 105. Diagram showing \ position of 

 leaves. The leaves numbered according 

 to their genetic sequence. (After STRAS- 



BURGER.) 



expressed as a fraction of the circumference, the DIVERGENCE. 

 different in different kinds of plants. 



It is 



In the case of verticillately-arranged leaves the angle of divergence of a whorl 

 (Fig. 106) is the circumference divided by the number of leaves, which is usually 

 the same in each whorl. The members of successive whorls do not stand 

 immediately above one another but alternate, so that the members of one whorl 

 come above the intervals between those of the whorl below (Fig. 99, 106). The 

 result of this arrangement, combined with the equality of the angle of divergence 

 in each whorl, is that the leaves of such a shoot are arranged in twice as many 

 vertical rows as there are members in each whorl (Fig. 106). These longitudinal 

 or vertical ranks are termed ORTHOSTICHIES. A frequent case of verticillate 

 arrangement is that of whorls of two members (Figs. 99, 106). In this arrange- 

 ment, which is termed DECUSSATE, the angle of divergence is 180 ; the divergence 

 is thus ^, and there are four orthostichies. If there are three members in a 

 whorl the angle of divergence is 120, the divergence ^, and there are six 

 orthostichies. 



