1 8 PLANTS 



which the indentations of the margin extend completely to the 

 midrib, thus producing a double scries of leaflets ranged along 

 a common midrib. 



29. When the divisions of the blade are all distinct so that 

 each resembles a miniature leaf, the leaf is said to be compound. 

 The divisions are then called leaflets and the common leaf- 

 stalk is the rachis. 



Phyllotaxy 



30. Leaves are usually arranged on the stem in a definite 

 order. On a vertical shoot there are two or more vertical ranks 

 of leaves. When there are two leaves at the same level they 

 are opposite, and each pair crosses the pair above or below at 

 right angles, making four vertical ranks of leaves. Sometimes 

 there are three or more leaves at the same level, forming a 

 whorl. If there is only one leaf at each level the leaves are said 

 to alternate. In this case every 2nd, 3rd, 5th, 8th or 13th, 

 etc., leaf, as the case may be, is in the same rank and there will 

 be 2, 3, 5, 8, or 13, etc., ranks respectively. This order is in 

 reality a spiral one, for if a line is drawn from one leaf to the next 

 higher one in the nearest direction and continued in this way it 

 will describe a spiral around the stem. This methodical ar- 

 rangement of the leaves evidently gives each leaf a maximum 

 of elbow room with respect to its fellow^s, and tends to equalize 

 the conditions of light and shade. 



31. The number of leaves which may receive sufficient light 

 exposure on a stem of a given length depends on (i) the size of 

 the leaf — a few large ones will shade each other as much as a 

 large number of small ones, (2) the shape of the blade — long, 

 narrow leaves or finely divided ones may be set more closely 

 than broad and entire leaves, (3) the length of the petiole — 

 other things being equal a long petiole will give the leaves 

 more room than a short one and consequently long petioles 



