552 



Prof. W. C. "Williamson. On the [June 17, 



being destined for each branch. One of these halves of the vasculo- 

 medullary axis displays, with the utmost distinctness, the characteristic 

 crescentic commencement of an exogenous zone, whilst the other half 

 retains its primary non-exogenous state. The latter condition thus 

 belongs to the Lepidodendron vasculare of Binney, whilst the former 

 as clearly represents the Sigillaria vascularis of the same author, and 

 the Sigillarian character of which is recognised by M. Renault. We 

 thus have in one stem two branches, one of which, according to the 

 views of the French savant, is a Cryptogamic Lycopod, and the other a 

 Gymnospermous Sigillaria. The remarkable peculiarities characterising 

 the central axis of these specimens makes it absolutely certain that 

 they all belong to one species of plant. 



The typical Lepidodendron Harcourth is then examined in a similar 

 manner. In the details of its organisation it differs materially from 

 L. Selaginoides, nevertheless as its growth progresses it displays typi- 

 cally similar changes. It attains to much larger dimensions than the 

 latter plant does before developing its exogenous zone, corresponding 

 in this respect with the Arran plant. Its earlier changes are chiefly 

 seen in the rapid development of the bast or prosenchymatous layer of 

 the outer bark and in the increase in the size and number of the vessels 

 constituting its vasculo-medullary cylinder or medullary sheath — the 

 ' : etui medullaire" of Brongniart ; but in more advanced specimens a 

 cylindrical zone of centrifugally developed vascular wedges begins to 

 make its appearance in a quasi- cambian zone of the cells of the inner 

 bark, these cells being arranged in more or less regular radiating lines. 

 In this state the rudimentary vascular zone corresponds very closely to 

 what is seen in young stems and roots of some Cycads. 



The author shows that, contrary to the views of M. Renault, very 

 marked changes take place in the development of the vascular bundles 

 destined for the secondary branches of the plant. In the first instance, 

 each of these is but a concavo-convex segment of the entire vasculo- 

 medullary cylinder, whose detachment leaves a large gap in the con- 

 tinuity of that cylinder, which, however, soon becomes closed again by 

 the convergence of the disconnected ends of the broken vascular 

 circle. The concavo-convex, detached segment undergoes a similar 

 change. Its two extremities meet, and before it escapes from the 

 outermost bark, it has assumed the cylindrical form of its parent stem. 



The rootlets of Stigmaria ficoides, now well known to belong alike 

 to Lepidodendron and to Sigillaria, present some peculiarities of 

 structure which are only found in the Lycopodiaceas and the Ophio- 

 glossse, amongst living plants. 



The vascular bundle in the interior of each Stigmarian rootlet is 

 enclosed within a very regularly circular cylinder, composed of the 

 cells of the innermost bark ; but the position of the bundle in relation 

 to the cylinder is always, unless accidentally disturbed, an eccentric 



