CHAP. II. 



LEAVES. 97 



is also eight. The position of the several heginnhu/s of the 8 

 spirals (viz. on Nos. 1. to 8.) is shown in C; and in D we 

 have the numbers (169. 106. 43. &e.) which respectively 

 begin th.e second series of each spiral. 



To discover the primary spiral, we may fix on any scale 

 as a point of departure (No. 1.), and then, by numbering the 

 scales on two of the secondary spirals (as 1. 9. 17. &c. and 

 1. 6. 11. &c.) which proceed in opposite directions, we may 

 afterw-ards very readily place the numbers on all the scales. 

 The easiest method of obtaining the common differences (viz. 

 8 and 5), for the purpose of numbering the scales in the two 

 cases selected, is to draw- a circle roimd the cone, and count 

 the nvnnber of each of the two kinds of spirals intersecting it 

 (which will be 8 of the first and 5 of the second). When a 

 secondary spiral perfects a complete coil (as 1. 9. &c. 161. 

 169.), tlie ntmiber of the spirals of the same kind is readily 

 seen; but the former mode for obtaining this number will 

 apply equally well to cases where the cone is too short for the 

 coils to be completed." 



In their normal state leaves are obviously distinct, both from 

 each other and from the stem. But, in some cases, adhesions 

 of various kinds occur, and give them a new character. Thus, 

 in Cardui, and many other thistle-like plants, the elongated 

 bases of the leaves adhere to the stem, and become what is 

 called decurrent. In Bupleurum perfoliatum the lobes of the 

 base of the leaf not only cohere with the stem, but, projecting 

 beyond it, grow together, so as to resemble a leaf through 

 which the stem has pierced : this is called being perfoliate. 

 Frequently two opposite leaves grow together at the base, as 

 in Caprifolium perfoliatum ; to this modification the latter 

 term is often also applied, but that of connate is what more 

 properly belongs to it. 



The anatomical structure of the leaf is this : — From the 

 medul'ary sheath diverges a bundle of woody tissue, accom- 

 panied by spiral vessels : this passes through the bark, and 

 proceeds, at an angle more or less acute, to a determinate 

 distance from the stem, branching off at intervals, and, by 

 numerous ramifications, forming a kind of network. At the 

 point of the stem whence the bundle of fibro-vascular tissue 



H 



