56 ANATOMY OF LEAVES. 



Biimet, Roses, &c. In these plants the terminal leaflet is produced 

 before all the others, the pair of leaflets nearest the apex of the leaf 

 next make their appearance, then the second pair, the third, and so on, 

 from the apex to the base. All digitate and radiate leaves belong to 

 the centripetal mode of formation. In PotentiUa reptans, &o., not 

 only do the leaflets grow from the top downwards, but their secondary 

 nervures and toothings appear in the same way. In plants belonging 

 to the MiXEB TTPE, the two preceding modes of development are combined. 

 The lobes of the leaves of Acer platanoides, &c., and the midribs of 

 those lobes which are digitate, form from above downwards ; the lower 

 lobes are produced last, but the secondary nervures and the toothings 

 are developed like those of the Lime-tree. The paeallbl roEMAiioif is 

 common to many Endogens. All the nervures are formed in a parallel 

 manner ; but in this, as well as in the case of dicotyledonous plants, 

 the sheath is the first that makes its appearance (Carex). The leaf 

 lengthens more especially by the base of the blade, or that of the 

 petiole when it exists (Chamserops) ; the sheath, often extremely small, 

 does not increase in growth tiU a later period ; the same holds true with 

 regard to Exogens when they have a sheath. As regards the growth of 

 leaves, which has been confounded with their mode of formation, 

 M. Trfioul has shown that all leaves which are furnished with sheaths, or 

 those which are very much protected by having their lower portions 

 enveloped with other organs, grow most by the base ; on the other 

 hand, those of which the whole petiole is exposed to the air at a very 

 early period, in oonseq^uence of the stem lengthening, grow much more 

 towards the -upper part of the petiole (Tropasolum majus, ^sculus, &o.) 

 Nevertheless, there is a short space near the insertion of the petiole in 

 the blade where the increase in length is less than a little lower.down. 



Considered with respect to its anatomical structure, a leaf is 

 an expansion of the bark, consisting of cellular substance, 

 among which are distributed veias. The former is an expan- 

 sion of the rind ; the latter consist of woody matter arising 

 from the neighbourhood of the pith, and from the liber. As 

 the tissue forming veins has a double origin, it is arranged in 

 two layers, united firmly during life, but separable after death, 

 as may be seen in leaves that have been lying for some time in 

 water. Of these layers, one is superior and arises from the 

 neighbourhood of the pith, the other inferior and arises from 

 the liber ; the former maintains a connection between the wood 

 and leaf ; the latter establishes a communication with the bark. 

 Since sap, or ascendiag fluid, rises through the wood, and more 

 especially the alburnum, afterwards descendiag through the 



