6 A TEXT-BOOK OF BOTANY 



will form a spiral winding about the stem (Fig. 1, ^4). As a 

 consequence, leaves with this arrangement are said to be 

 spiral, though they are still often called alternate. On 

 account of this spiral arrangement, two successive leaves are 

 in different vertical planes, and the danger of the upper leaf 

 shading the lower is reduced. In other plants two or more 

 leaves appear at each node; and as an imaginary line con- 

 necting their points of origin forms a circle about the stem, 

 the arrangement is called cyclic. Very commonly, however, 

 when two leaves appear at a node they are said to be 

 opposite (Fig. 1, B); and when more than two appear they 

 are described as whorled (Fig. 1, C). The cycle of leaves at 

 one node does not stand directly over the cycle at the node 

 below, but over the spaces between the lower leaves, the 

 danger of shading being reduced as in the case of the spiral 

 arrangement. In fact, the cyclic arrangement differs from 

 the spiral only in having two or more parallel spirals. 



9. Regions. The conspicuous part of a leaf is the ex- 

 panded portion known as the blade, and often the leaf is all 

 blade. In many cases the leaf has a stalk (petiole) which 

 bears the blade more or less away from the stem; and in 

 certain groups of plants a third region is evident, usually 

 consisting of a pair of more or less blade-like appendages 

 (stipules) on the petiole where it joins the stem (Fig. 2, A). 

 As might be expected, the essential part of the leaf is the 

 blade, and ordinarily when the word leaf is used it refers to 

 the blade. 



10. Venation. Upon examining an ordinary leaf, the 

 blade is seen to consist of a green substance through which 

 a network of veins is variously distributed The larger 

 veins that enter the blade send off smaller branches, and 

 these send off still smaller ones, until the smallest veinlets 

 are invisible. This is plainly shown by a skeleton leaf; 

 that is, one which has been so treated that all the green 

 substance has disappeared, and only the network of veins 



