62 The Living Plant 



and new branches from buds in the angles between stems and 

 leaves, a position which has the advantage of nearness to the 

 manufactories of food. This brings us to consider the causes which 

 determine the arrangement of leaves on the stem, a curious matter, 

 scientifically called phyllotaxy, and once discussed more commonly 

 than now in botanical books. Leaves do not originate on the stem 

 at hap-hazard, as may seem the case on some slender branches, 

 but in quite definite and even mathematical order, as rosette- 

 like plants, cones, and some other very compact structures sug- 

 gest. Two primary systems of leaf-arrangement are possible, 

 and occur. The simplest is the opposite (or whorled] system, in 

 which two leaves stand at the same node exactly opposite one 

 another, as occurs for example in the Mints, (figure 16, A}, in 

 which case the next pairs above and below stand at right angles 

 and thus cover the space left by the first set, producing four vertical 

 rows often in remarkable symmetry, as our common cultivated 

 Coleus illustrates. This, with the other arrangements, is shown 

 diagrammatically in figure 16, where the reader is supposed to 

 look down from above on the stem, which is imagined to be tel- 

 escoped, so to speak, Chinese lantern fashion, to a single flat plane, 

 as indeed the stems actually are in the buds. In some kinds, 

 three instead of two leaves stand at a node, or four or five, or 

 more, producing a regular whorl, but in all such cases, illustrated 

 for instance by large Lilies (figure 16, B), the leaves in a whorl 

 are evenly spaced and cover the breaks in the whorls above and 

 below. This is the system prevalent in flowers, for, as everyone 

 will recall, the whorl of sepals covers the breaks in the whorl of 

 petals, with a similar arrangement in stamens and carpels. Thus 

 much for the opposite or whorled system; the other is the spiral, 

 in which only one leaf ever stands at a node, while the one on the 

 node next above or below stands part way around the stem, 

 the successive leaves falling always into a regularly-ascending 

 spiral. Now this space around the stem from one leaf to another is 

 a definite fraction of the circumference; in some plants it is ^, 



