STRUCTURE.] DEVELOPMENT OF COMPOUND LEAVES. 289 



"Development of a Compound Leaf . A compound leaf, when 

 its leaflets and stipules become first visible, is like a crenate 

 or lobed leaf, inserted on the axis by a base more or less en- 

 larged. As the crenels or lobes which correspond with the 

 leaflets are no longer in direct relation with the axis of the 

 bud, from which they are separated by the rim, which is in 

 the place of a common lamina, it is probably in this lamina 

 that the mother-cells are formed, which serve for the growth 

 of the crenels. The latter gradually become leaflets. Whilst 

 this transformation is going on, the leaf, which was at first 

 simple, becomes lobed, then pinnatifid, then pinnate, then, 

 and not before, the leaflets begin to be developed ; they behave 

 with respect to the rachis, as the originally simple leaf did 

 with respect to the principal axis. 



The blade of the leaf is first formed, and moreover in such 

 a way, that the young lamellae fold up so as to produce a 

 medial furrow, extending from their apex to their base. This 

 is the only mode of development found in really compound 

 leaves. As to the stipules, if they remain immediately next 

 the axis, they extend their blade over its nucleus (as for 

 example in Baptisia minor and Melianthus major), and this is 

 the real difference between them and leaflets. 



When the blade of the leaflet is tolerably advanced in its 

 development, the petiolule begins to appear, either as a con- 

 traction, or as a small cylindrical stalk; from this time forward 

 the development of the leaflet is no more retarded. 



When there are any stipels, their development begins at 

 the same time as that of the partial petioles. Their symme- 

 trical development is often interfered with by their position 

 relative to the petiole and partial petioles ; whence they are 

 often abortive, or they are only developed as prickles or very 

 narrow leaflets. 



The most remarkable change to which a compound leaf is 

 subject is caused by the development of the common petiole. 

 Whilst the leaflets become more and more distinct, as lobes 

 or crenels of the lamina, the remaining portion of the latter 

 organ constitutes an elongated and more or less cylindrical 

 body, proceeding from the nucleus of the axis, and support- 

 ing the leaflets. In other cases a very short stem only 



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