34 THIRD REPORT — 1833. 



formed in the manner that has been supposed. According to 

 him, the new matter from which the wood results is not a mere 

 addition of new matter to the centre, but consists of bundles 

 of wood, which, originating at the base of the leaves, take first 

 a direction towards the centre, and then a course outwards 

 towards the circumference, forming a curve ; so that the stem 

 of a Palm is, in fact, a mass of woody arcs intersecting each 

 other, and having their extremities next the circumfei-ence of 

 the trunk. I regret that I have not been able to consult Dr. 

 von Martius's splendid work on Pahns since this Report was 

 commenced, and that I am therefore unable to state upon what 

 evidence Dr. Mohl has rested his theory. 



The same writer has stated* that Cycadeoe — that singular 

 tribe, which is placed, as it were, on the boundary line between 

 cellular and vascular plants, — are not in a great measure desti- 

 tute of vessels as is commonly supposed, but, on the contrary, 

 are composed exclusively of spiral vessels and their modifica- 

 tions, without any mixture of woody fibre. I have already ad- 

 verted to this hypothesis in speaking of the same author's state- 

 ment, that the dotted tubes of Cycadece are a slight modi- 

 fication of vasa scalariformia. Dr. Mohl is also of opinion 

 that Cycadece are not exogenous in their mode of growth, as 

 seems to be indicated by their appearance when cut, and by 

 their dicotyledonous embryo, but that they are moi'e like 

 Palms in their manner of forming their wood, which is essen- 

 tially endogenous. He asserts that the stem of Cycadece, in 

 regard to its anatomical condition, must be considered inter- 

 mediate between that of Tree Ferns and Coniferce, just as their 

 leaves and fructification undoubtedly are. He states that in 

 CycadecB a body of wood is gradually formed of the fibres con- 

 nected with the central and terminal bud ; that so long as this 

 original wood is soft, and capable of giving way to the fibres 

 that are continually passing downwards, no second cylinder of 

 wood is formed ; but in time the original wood becomes hard- 

 ened, and then the new fibres find their way outward and down- 

 ward, collecting into a second cylinder on the outside of the 

 original wood. It is obvious that this explanation is not so sa- 

 tisfactory as could be desired ; for, in the first place, such a 

 distinction between Cycadece and Exogence as that which Dr. 

 Mohl states to exist, is verbal rather tlian real, since he admits 

 that the second cylinder of wood is formed externally to the 

 first ; and secondly, it is obvious that if that structure which is 

 represented in the 21st plate of the third volume of the Hortus 



* Ueber den Bau des Cijcadeen Stammes und sein Verhaltniss xu den Stamin 

 der Coniferen und Baumfarrn. 4to. Munich, 1832. 



