STRUCTURE.] ORIGIN OF COMPOUND LEAVES. 279 



are quite contrary to those of M. Regel (Linnsea, xvii., second 

 part, 1843), who thinks that stipules, especially of Mono- 

 cotyledons, grow before the leaves. 



"Formation of a Compound Leaf . The formation of com- 

 pound leaves differs very much from that of simple ones. In 

 both, however, the apex is the part first formed. 



A compound leaf resembles at first a simple, sessile, undu- 

 lated or crenated leaf; it then becomes lobed, then pinnatifid, 

 and lastly pinnate. 



The leaf of Baptisia minor consists of three shortly petiolate 

 leaflets, and of two oblongo-lanceolate stipules. The summit 

 of the axis of the leaf-bud is a roundish cone ; its form is, 

 however, considerably modified in consequence of the deve- 

 lopment of the leaves. The tumour which appears first, and 

 which corresponds with the middle leaflet, grows from the 

 top of the axis, so that the latter has two unequal projections 

 separated by a slight depression. This tumour then elongates 

 obliquely, and is by degrees changed into an ovoi'd, sessile 

 scale, surrounding nearly the whole axis, and having three 

 bosses. The two inferior bosses, which are the smallest, are 

 opposite to and correspond with the two lateral leaflets. 

 The scale increases in size, and applies itself to the axis by 

 its three bosses, which are joined by two others, the stipules ; 

 the scale now completely surrounds the axis. The bosses 

 become more and more distinct ; and we at last find a three- 

 lobed leaf, sheathing the axis on one side, and bearing two 

 stipules on its opposite side. All the succeeding transforma- 

 tions are perhaps attributable to the influence of the common 

 petiole ; for from this time forward none of the perceptible 

 parts of the leaf are any longer in direct conjunction with 

 the axis; between them is seen a shapeless cellular mass, 

 which subsequently becomes the common petiole ; the mar- 

 ginal incisions penetrate more and more into this mass, and 

 a trifid leaf is eventually formed. The rest of the mass 

 forms the petiole, and partial petioles. 



The axis of the leaf-bud of Medicago sativa is a roundish 

 cone. At the time that the leaves are formed we find on its 

 periphery a little cavity above a rather rounded projection. 



