ItANJJNCULACEJE. 69 



bundles, separated from one anotlier by as many medullary rays, 

 which are continued into the cortical parenchyma. When adult, only 

 a single layer of liber envelopes this wood, all the rest of the bark 

 having fallen, or else there are a certain number of plates of liber, 

 crescent-shaped in transverse section, more or less ready to fall, 

 owing to the development of a layer of cells on the inner side. 

 Hence it is only when young that tliere are outside the bark an 

 epidermis and a parenchyma, whose cells have green contents. Later 

 on, the exfoliated branches of most of the genus only present bundles 

 of fibres on the surface, separated from one another by the peripheral 

 edges of the medullary rays.' 



* The evolution of these steins should be 

 studied closely and in detail from a histological 

 point of view. In a young hexagonal axis (the 

 sides may increase in number when the leaves 

 become verticillate) we only see a neai-ly homo- 

 geneous tissue witliin a hairy epidermis. Later 

 on appear six equidistant fibro-vascular bundles, 

 each in two parts ; the one, fibro-vascular, be- 

 longs to the wood ; the other, which represents 

 the cortical fibres, is at some distance from the 

 former, from which it is separated by a thick 

 zone of formative cells. The six medullary rays 

 are also very large, and in the thickness of each 

 of these we see formed a little later a younger 

 fibro-vascular bundle. This set of buntUes alter- 

 nate with the first and afterwards grow so as to 

 have equal, or nearly equal dimensions, with them. 

 When the twelve bundles have come into contact 

 with, and compressed one another, and are all 

 triangular in horizontal section, the wood pre- 

 sents twelve linear medullary rays, separating 

 them from one another. The twelve bundles of 

 cortical fibres increase so that the transverse 

 section of each of them soon becomes a crescent 

 with its convexity outwards. The formative zone 

 is then represented, not by a ring, but by twelve 

 cellular crescents, moulded in the concavity of 

 the fibrous crescents. Still later, fibres appear 

 within this cellular crescent, also arranged in 

 an arc concentric with the preceding (besides 

 the fact well established by Giuou de Bu- 

 ZAEEINGUES, of deduplication from without in- 

 wards, two fibrous crescents forming within each 

 of those of the first gener:ition, two again within 

 each of the second generation, and so on). Thus 

 we have in each segment of the transverse sec- 

 tion two fibrous concentric arcs, separated by a 

 cellular crescent. It is here tluit the separation 

 takes place ; the cells wither and leave the inner 

 crescent, while still adhering to the outer crescent 

 which thej' bring with them. Sucli is the cause 

 of the exfoliation. We should add that under the 

 epidermis, along the projecting angles of the 



stem, the cortical cellular tissue undergoes a 

 difl'erent transformation into elongated elements 

 with whitish thickened walls. These outer 

 bundles also fall later on, with the detached 

 fibres of the bark. The same exfoliation occurs 

 in Pmonia and Xanihoriza, but it is less evi- 

 dent because of the number and small size of 

 the bundles, which appear to have a less regular 

 arrangement in transverse section, on account of 

 tlie leaves being alternate. In the Kii/ellce with 

 sulcate stems (and especially in Garklella), in 

 ThaUctrum, in certain Aconites (especially the 

 sarmeutosc species), the fibro-vascular bundles of 

 the wood itnd bark are similarly organized, and 

 in Garklella are distinctly seen the projecting 

 angles under the epidermis where the outer tissue 

 of the bark also becomes thickened, elongated, 

 and fibroid. Often in these plants the cortical 

 fibrous portion of each bimdle begins to separate 

 from the deep parts ; the death of the branch 

 stops the exfoliation. In several species of 

 Clematis also, with herbaceous annual branches, 

 like C. iubulosa, the exfoliation has not time to 

 appear. In the young branches of C. montana, 

 which die in our country after tlieir season of 

 vegetation, there is no exfoliation of the bark, 

 and the bundles touch and are united into a sort 

 of ring in the cortical portion, so that the liber 

 on the whole, nearly assumes the tubular form 

 it presents in most woody plants. In some 

 species (in this respect intermediate), t'le fibrous 

 crescents of the bark are much multiplied before 

 the occurrence of desquamation, as are the cel- 

 lular crescents which line them ; we then see at 

 the same time very many of these little arches 

 differing greatly in age, of which the outer ones 

 alone (which are very old) commence to separate 

 from the younger ones. H. Mohl called at- 

 tention to the fact that the cells of the pro- 

 scnchyma in Clematis are shorter than is usually 

 thought, being only from -f^" to ^"' (^Ann. Sc. 

 ^'at., ser. 4, v. 144.) 



