o 4 NATURAL HISTORY OF PLANTS. 



this question in 1837,' found that in Menispermum canadense, for 

 instance, a young branch in its first year, while the tissue is still 

 herbaceous, presents the same general organization as that of any 

 other Dicotyledon ; but that " this structure, after succeeding years 

 of o-rowth, differs widely from that of other deciduous Dicotyle- 

 dons. In fact, on observing a two-year-old branch, no new layer is 

 found, but instead of this, an elongation of each vascular bundle 

 whose obovate form is still more marked ; at its outer part are still 

 distinguishable the semilunar layer of rudimentary wood, and the 

 layer of liber of the same form. This liber is the same unchanged ; 

 but the formerly rudimentary wood is now perfect wood and has 

 produced afresh a layer of cambium. So this mode of growth 

 goes on indefinitely, without the deduplication of any of the fibro- 

 vascular bundles or the addition of any fresh ones, their number 

 remaining unaltered. " Each of these bundles has grown chiefly at 

 its outer parts, to which new fibres and vessels are constantly added. 

 During all this the liber remains stationary, and in a stem of consi- 

 derable growth only very small bundles are formed outside the 

 segments of the woody zone. In a non-climbing species with 

 persistent leaves, like Cocculus lauriflorm, the primitive structure of 

 a young branch is the same, and the fibro- vascular bundles are 

 narrow in proportion to the progressively increasing breadth of the 

 medullary rays. After several years have passed there make their 

 appearance in the cortical parenchyma, outside the first fibro-vascular 

 bundles and alternating with them, an equal number of fresh 

 bundles ; these, however, differ histologically, possessing no spiral 

 vessels and having no liber outside them. Later on, a third 

 formation of vessels takes place, alternating with the second, 

 and of identical structure. Thus may further appear in turn 

 a fourth circle, a fifth and so on. 2 The growth of the stem 



1 In Arch. Mus., i.154, t. 10. cells of the primary bark, or else from these and 



2 Tr£cul was the first to explain (in Ann. Sc. from their immediately external neighbours." 

 Nat., ser. 3, xix. 265) the formation of these He has also shown that " the secondary vascular 

 secondary bundles, saying, " Tbey are the cells bundles are never connected with the leaves." — 

 at a certain depth in the youngest utricular layer See also on this question NiGELi, Beitr. zu Wiss. 

 of the bark, which have first enlarged and then Bot., i. 16. In this memoir the Menispermads 

 been divided by septa." Radlkofeb (in Flora are taken as the type of the Dicotyledons with 

 (1858), 193 ; in Ann. Sc. Nat., ser. 4, x. 164) successive limited zones of cambium in the 

 has also referred their origin to a new cambium " protenchyma." 



formed at a certain age " from the innermost 



