CH. IV, 5] ARRANGEMENTS OF LEAVES 



139 



The mode of formation of stem and leaves within buds 

 is revealed by longitudinal sections. In an illustrative case 

 (Fig. 92), one can see very clearly the blunt conical end of 

 the stem, composed of small tightly-packed cells in process 

 of formation through new cell-divisions, while backwards 

 the cells are evidently beginning to elongate with the 

 lengthening of the stem. The first visible trace of a leaf is 

 found in the enlargement of a superficial cell, which soon 

 divides ; the resultant cells again divide and, including both 

 epidermis and cortex, gradually push out in a flat projection, 

 -the leaf. These leaves enlarge steadily, thus making a 

 perfect gradation from those just appearing to those fully 

 formed. As they develop, the nodes on which they stand, 

 at first cloSe together, are carried apart by lengthening of 

 the internodes, and the tissues gradually pass from the 

 meristematic, or embryonic, to the differentiated or mature 

 condition. In this process, however, the areas of tissue in 

 the axils of the leaves remain meriste- 

 matic, thus originating the axillary 

 buds. 



5. THE ARRANGEMENTS OF LEAVES 

 ON STEMS 



u 



Leaves develop upon stems not at 

 haphazard, but in definite math- 

 ematical order. This definiteness 

 of arrangement, called scientifically 

 PHYLLOTAXY, while sometimes obscure, 

 is often strikingly manifest to the eye. 



When two leaves occur at a node, 

 they are always OPPOSITE to one an- 

 other, and each pair stands at right 

 angles to the pairs above and below, 

 thus forming four ranks on a vertical FIG. 93. The opposite 



stem is thp Polpm of our ffardpns arrangement of leaves, as 

 ' c r o^ 1 [lb > illustrated by a museum 



and the Mint family in general, well model. 



