OF THE LEAVES. 761 



Development of Leaves is due to the admirable investigations of 

 De Mercldin and Trecul, and we shall content ourselves, there- 

 fore, in giving a general abstract of their views upon this subject. 

 The following is a simimary of De Mercklin's conclusions : — 



'•' All leaves are produced on an axis, and their first form is 

 that of a tumour. The lobes, segments, or leaflets on the lower 

 half of a completely formed leaf are produced from the axis, 

 after the lobes, segments, or leaflets on the upper half. 



" The original tumour corresponds with the apex of the leaf, 

 or with the summit of the common petiole. 



"In all leaves, the blade and top of the petiole are formed be- 

 fore the stipules and the lower part of the petiole. 



" The formation of compound leaves consists of two stages : 

 first that of a simple leaf; then that of a pinnate leaf. It is 

 not very probable that the second owes its origin to the axis of 

 the leaf bud as the first does. The petiole (either of a simple 

 or of a compound leaf) ought, whether we regard its position 

 relative to the axis or its anatomical structure, to be considered 

 as an immediate elongation of the axis ; it certainly has a great 

 influence on the formation of the leaf. 



" The stipel is formed after the point of the leaflet which it 

 accompanies; its development is generally much slower than that 

 of a stipule. 



" All the parts of a leaf are symmetrical from their birth, and 

 the rudiment of each leaf is a body symmetrical in its relation to 

 the axis. 



" The young leaflets of all compound leaves are always 

 opposite. 



" All the parts of a rudimentary leaf are capable of develop- 

 ment. This development generally proceeds from the apex to 

 the base of the leaf, and is greater and stronger towards the 

 latter. The development takes place in all directions, and 

 predominates in determinate directions. 



" The blade of a leaf is first developed. Leafy lamellse are ex- 

 tensions of it, whether they are eqiiilateral or inequilateral. 



"Teeth and crenels appear to be owing to the development of 

 certain series of cells from the edge of a leaf. No trace of 

 them is to be found in very young leaves, the blades of which 

 are beginning to be formed. 



" Stipules of Dicotyledons, in consequence of the great longi- 

 tudinal development of the petiole, appear as organs distinct 

 from the blade. The rapidity of their development is probably 

 due to their proximity to the axis. Their blade is developed, 

 covering the axis or other organs. 



" The petiole is principally developed in one direction ; of all 

 the parts of a leaf, it is that which grows the most in proportion 

 to its original size. 



" Although most of these views are founded on facts, yet they 



