230 



LEPIDODENDREAE. 



living plants, as *in the stems of Tecoma radicans, inevitably results in the 

 bursting of the outer ring of wood, no signs of which have ever been 

 observed in Lepidodendron. We must indeed have recourse to assumptions 

 before the formation of secondary wood, in order to understand the increase 

 in size of the central strand, as Williamson conceives it. First of all, if his 

 view is correct, there must always have been parenchyma-cells present 

 between the tracheides, to be the starting-point of the formation of the inner 

 parenchyma in the originally solid strand. Next, these parenchyma-cells 

 must have been able subsequently to give rise to more tracheides, for the 

 increase in the thickness of the peripheral ring could not otherwise be 

 explained ; and lastly, cells of the kind must have been introduced into the 

 ring itself, to render its surface-growth possible. All this is however quite 

 possible ; the single parenchyma-cells from which the development proceeds, 

 may easily have been overlooked in the preparations, and it is indeed in 

 favour of this supposition, that Williamson 1 has in a few cases met with the 

 central parenchyma apparently in the state of meristem. The preparation 

 here cited, which his kindness enabled me to examine, certainly gives quite 

 this impression. If therefore we allow the possibility of the increase in 

 volume up to the time when the central strand is inclosed in the secondary 

 wood, it is entirely excluded after that time, as has been already said. 

 And this shows that the way in which Williamson has arranged his prepara- 

 tions to represent a course of development cannot be right. It is to be 

 remembered, that we must not simply compare the terminal ramifications of 

 the head of a tree with the still young and growing ends of the main shoot 

 or of its subordinate branches. The central strand in the main shoot and 

 in the branches may and will have had a very different diameter at the 

 beginning of the growth in thickness from that of the later generations of 

 branches, the last of which may have had no growth of the kind. If this is 

 so, and I have no doubt whatever about it, every arrangement of the single 

 stages into a successive series is naturally precluded, for we must know to 

 what part of the branch-system each piece belonged, and this in the frag- 

 mentary condition of our remains we are unable to determine. 



The chief sources of our knowledge of the anatomy of Halonia are the 

 works of Dawes 2 , Binney 3 and Williamson 4 . From these we learn that 

 there is on the whole essential agreement in structure between this form and 

 the type of Lepidodendron Harcourtii, or with that of the Burntisland 

 plant ; there is indeed no growth in thickness, but it is quite intelligible that 

 there should be no such growth in a branch serving only as a fructification- 

 stalk. Only we know nothing of the character of the outer cylinder of the 

 rind, since this has never been observed in the specimens with the structure 



1 Williamson (1), xn, t. 33, f. 20. a Dawes (1). 3 Binney (1), III. * Williamson 



(1), n, p. 222 and notes, and xn, p. 466; t. 32, f. 21. 



