8 INTUODDCTION. 



ing from the simple leaf, as already described (Fig. 4), which has but a 

 single blade. The separate blades of a compound leaf are called leaflets 

 and the foregoing terms, used of the form, margin, etc., of the simple 

 leaf, are equally applicable to the leaflet. They may be sessile on the 

 main leaf-stalk, or they may be supported on stalklets of their own 

 called petiolules and these may or may not be jointed or articulated to 

 the main stalk, just as that is to the branch. There are two classes of 

 compound leaves, corresponding to each other as do the pinnately and 

 palmately- veined simple leaves. They are as follows: 



39. Pinnately compound, when the leaflets are arranged along the 

 common leaf-stalk here called the rachis like the vanes of a feather 

 (whence the name), or like the veins of a pinnately-veined simple leaf. 

 When the rachis terminates with a single leaflet the leaf is said to be 

 oddly or unequally pinnate; when with a pair of leaflets it 

 is said to be evenly, equally or abruptly pinnate. 



40. Palmately or digitdtely compound, when the leaflets 

 radiate from the summit of the leaf -stalk as do the veins 

 of the palmately-veined simple leaf. (-Fig. 18. Compare 

 with Fig. 5.) 



41. Sometimes the divisions of a compound leaf instead 

 of being simple leaflets are themselves compounded, when 

 the leaf is said to be twice compound, in which case one of 

 the main divisions is called a pinna (pi. pinnae) and its 

 subdivisions are called leaflets; when these are compounded 

 they are called pinnules and their subdivision leaflets, and 

 the whole leaf is said to be thrice compound pinnately 

 or palmately as the case may be. The subdividing may 



FIG. 17. go on still further, and on the same plant it may be varia- 

 ble. For such leaves the term decompound is used. 



42. By the use of the prefixes hi, tri, etc. (from L. bis, twice, ter, 

 thrice, etc.), and the adjective foliate (L. folium, leaf), the number of 

 leaflets of a compound leaf may be designated, and the leaf is said to be 

 pinnately or palmately bifoliate, trifoliate (or ternate), etc., as there are 

 two, three, etc., leaflets. Using the same prefix to designate the num- 

 ber of times compounded a leaf is said to be bipinnate or tripinnate 

 when twice or thrice pinnately compound, etc. 



43. The Arrangement of Leaves on the 

 Stem (technically known as pliyllotaxy) is not by 

 chance, as the casual observer might suppose. The 

 place for the appearance of each leaf is determined 

 beforehand, and by certain laws very exact and beau- 

 tiful in their workings. We cannot,however,consider 

 them to any extent here. It must suffice in this 

 place to note the fact that but one leaf springs 



from the same point on the stem, and there are p IG 



primarily two systems of arrangement, as follows: 



Alternate, when the leaves spring one from a joint or node, as it is 

 called and alternately from opposite sides of the shoot. (Fig. 19.) 



Fig. 17. A Pinnately compound Leaf. 

 Fig. 18. A Palmately compound Leaf. 



