THEIR ARRANGEMENT. 141 



anticipate several of the leading points of the present chapter. As 

 to the general office of leaves in the vegetable economy, it has 

 been assumed that the leaf is an apparatus in which, under the 

 agency of sunlight, the sap is digested, and converted into the 

 proper nourishment of the plant (79, 114). As to their situation 

 upon the stem, it has been stated that they invariably arise from 

 the nodes (141), just below the point where buds appear (148). 

 So that wherever a bud or branch is found, a leaf exists, or has 

 existed, either in a perfect or rudimentary state, just beneath it ; 

 and buds (and therefore branches), on the other hand, are or may 

 be developed in the axils of all leaves, and do not normally exist 

 in any other situation. And finally, the relation of leaves to the 

 wood and the general structure of the stem has just been noticed 

 (224-231). From its natural connection with that topic, it will 

 be most convenient first to consider their arrangement on the stem. 

 This subject, which has of late been elaborately investigated, has 

 received the name of 



234. Phyllotaxis (from two Greek words, signifying leaf-arrange- 

 ment). We can here only briefly illustrate the general laws which 

 appear to regulate the arrangement of leaves on the stem, as man- 

 ifested in the several" modes which are of ordinary occurrence. 



235. The point of attachment of a leaf (or other organ) with the 

 stem is termed its insertion. 



236. In botanical descriptions, leaves are said to be alternate 

 (149), when there is only one to each node or phyton, as in Fig. 

 168, in which case the successive leaves are thrown alternately to 

 different sides of the stem : they are said to be opposite when each 

 node bears a pair of leaves (149, 231), in which case the two 

 leaves always diverge from each other as widely as possible, that 

 is, they stand on opposite sides of the stem and point in opposite 

 directions (Fig. 107, 104), or else they are verticillate or whorled, 

 when there are three or more leaves in a circle (verticil or whorl) 

 upon each node ; in which case the several leaves of the circle di- 

 verge from each other as much as possible, or are equably distrib- 

 uted around the whole circumference of the axis. The first of the 

 three is the simplest as well as the commonest method, occurring 

 as it does in almost every Monocotyledonous plant (where it is 

 plainly the normal mode, Fig. 168), and in the larger number of 

 Dicotyledonous plants likewise, after the first or second nodes. It 

 should therefore be first examined. 



