ABBANGEMENT OF LEA VE8. 



153 



as follows :* "New lateral members have their origin above 

 the centre of the widest gaps which are left at the cir- 

 cumference of the punctum vegetationis between the in- 

 sertions of the nearest older members of the same kind ;" 

 and no doubt this is one of the most important immediate 

 causes which determine where each new leaf is to arise. If it 

 be asked why, then, are not all leaves arranged alike, the 

 answer must be looked for in the differences in structure of 

 the puncta vegetaiiones. In cases where there is an apical 

 cell, the arrangement of the leaves may be directly traced to 

 its mode of division. In Phanerogams it is often clearly due 



Fig. 1306.— Cross-Fection of the leaf-ljud of the chostnut (Castanea vesca). l)', v', 

 ■the scale-like leaves jy^i/^j/s, etc., the rndimentiiry leapeK ; s^sK s^-s"^, etc., the 

 atlpules belonging to' the correspondingly numbered leaves. Magnified. — After 

 Hofmeister. 



to a difference in the size and form of the punctum vegeta- 

 tionis ; in Conifers and Composites, for example, it is com- 

 mon for a change in the arrangement to take pilace in pass- 

 ing from the foliage leaves to the bracts of the inflorescence 

 upon the same stem, the number of ranks in such cases 

 being greater on the larger axes. Doubtless some of the dif- 

 ferences can be explained only by taking into account, also, 

 the inherited j)eculiarities of the plant. 



* " Allgem. Morphol.," p. 483, and quoted in Saclis' "Text-Book," 

 p. 177. 



