APPENDIX. 83 



tion to the fundamental proposition on which Mirbel rests. 

 Mirbel says, because in the Palms the vascular bundles 

 are softer above than below, their lower portion is older, 

 they consequently grow from below upwards ; with equal 

 right he would assume the reverse if he split an internode 

 of Zea Mays. Moreover, if we remove the leaves from a 

 terminal bud of Phoenix, we find in its half-developed 

 leaves, which have attained a length of about 1 to 3 lines, 

 the part of the petiole projecting from the bud already 

 green and firmly lignified, while the base of the petiole 

 and sheath are uncoloured and very soft ; the vascular 

 bundles of the upper part are found completely lignified, 

 those of the lower portion still half gelatinous and trans- 

 lucent, in short, we find here between the upper and 

 lower ends of the vascular bundles (only in reversed order) 

 the same differences as in the vascular bundles of the stem. 

 Shall we conclude from this that the vascular bundles of 

 the leaves grow from above downwards, that the upper 

 part of them is so much older than the lower ? This no 

 one will wish to assert, since in the Palm-leaf, it must be 

 concluded, from the fact that leaflets still very small have 

 a sheath, that the growth of the leaf depends on an ex- 

 pansion of the very young leaf in all directions, and not 

 on a subsequent after-growth of its lower portion. From 

 these circumstances, it certainly follows that the greatest 

 differences of solidity and completion of structure occur 

 simultaneously in different parts of the same vascular 

 bundle, and may be caused by the transition from the 

 rudimentary to the lignified condition proceeding with 

 different rapidity in different parts of it, and going on 

 parallel with the unequal growth of the organs in which 

 the vascular bundle lies, since, in general, the upper part 

 of the leaf attains its full development first, and, in the 

 slowly-growing Palm-leaf, a long time before the lower 

 portion ; the same holds good in the vascular bundles of 

 its petiole, and the direction in which their lignification 

 proceeds, can by no means be regarded as an index of the 

 direction in which their rudiments made their appear- 



