OKGAXS OF NUTRITION. 



137 



verticil or ivhorl When leaves are opposite, the pairs as they 

 succeed each other frequently cross at right angles, in which 

 case thev are said to dtcussate\fig. 264), and the arrangement is 

 called decussation. When different whorls succeed each other 

 it also frequently happens that a somewhat similar arrangement 

 occurs, thus the leaves of one whorl correspond to the intervals 



Fig. 263. 



Fig. 264. 



Fig. 263. Whorled leaves of a species of Galium Fig. 264. Decussate 



leaves of the Pimelea decussata. 



of that helow it. There are however commonly great irregu- 

 larities in this respect, and in some cases the number of leaves 

 in the different whorls vary, by which their arrangement becomes 

 still more complicated. This is the case for instance in Lysi- 

 machia vulgaris. 



Only one leaf can arise from the same point, but it sometimes 

 happens that by the development of an axillary branch the in- 

 ternodes of which are scarcely developed, that all the leaves of 

 that branch may be brought in contact at their base, in Avhich 

 case they form a tuft or fascicle (fig. 265), and the leaves are 



Fig. 265. 



said to be tufted or fascicled. Such an 

 arrangement is well seen in the Bar- 

 berry and the Larch. That fascicled 

 leaves are thus produced is rendered 

 evident by the fact, that in the young 

 branches of the Larch the internodes 

 become elongated and the leaves con- 

 sequently separated from each other. 



The laws which regulate the arrange- 

 ment of leaves upon the stem have of Fig. 26.5. Fascicled leaves • 



late years been carefully investigated; of the Larch. 



and when we consider that all the organs of the plant which 



succeed the leaves are modifications of them, and follow similar 



