6 A TEXT-BOOK OF BOTANY 
will form a spiral winding about the stem (Fig. 1, A). Asa 
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 
