REPORT ON THE PHILOSOPHY OF BOTANY. 35 



Malabarlcus be correct, where the gtem of Cycas circinalh is 

 shown to have several concentric zones, precisely as in other 

 exogenous trees, it must follow that Dr. Mohl's explanation 

 would be still more inadmissible ; accordingly, this author dis- 

 credits the fact of the stem of Cycas circinalis having numer- 

 ous concentric zones. It is, however, certain, from the speci- 

 mens brought to England by Dr. WalUch, that the structure 

 of this Cycas is really such as is shown in the Hortus Malaba- 

 rlcus. It is nevertheless extremely well worth further inquiry 

 whether there is not some important but as yet undiscovered 

 peculiarity in the mode of forming their stem by Cycadece ; for 

 it must be confessed that growth by a single terminal bud, 

 after the manner of Palms, is not what we should expect to 

 meet with in exogenous trees. 



Professor Schultz of Berlin has indicated* the existence of a 

 group of plants, the structure of whose stems he considers at 

 variance v^ith all the forms at present recognised ; and to this 

 group he refers Cycadece : but the assemblage of orders which 

 he collects under what he calls the same plan of growth is so 

 extremely incongruous as to lead to no other conclusion than 

 that subordinate modifications of internal structure are of no 

 general importance, but are merely indicative of individual pe- 

 culiarities. 



Dr. Mohl further states, that Cryptogamic plants of the 

 highest degree of organization, such as Ferm, Lycopodiaceeet 

 Marsileacece, and Mosses, in all which a distinct axis is found, 

 have a mode of growth neither exogenous nor endogenous, but 

 altogether of a peculiar nature. In these plants, when once 

 the lower part of the stem is formed it becomes incapable of 

 any further alteration, but hardens, and the stem continues to 

 grow only by its point, which lengthens merely by the progres- 

 sive development of the parts already formed, without sending 

 downwards any fibrous or woody bundles, as both in exoge- 

 nous and endogenous plants. 



M. Lestiboudois, the Professor of Botany at Lille, distin- 

 guishes Monocotyledons from Dicotyledons, upon principles 

 different from those generally adntitted. According to this 

 writer, dicotyledonous trees have two systems, one, the central, 

 consisting of the medullary sheath and the wood ; the other, 

 the cortical, composing the bark. These two systems increase 

 separately, so that in Dicotyledons there are two surfaces of 

 increase, that of the central system, which adds to its outside, 

 and that of the cortical system, which adds to its inside : but 



* N'aturlic/ii'.i Sijstem des Pflanzenreichs nach seiner inncrcn Organixaiion. 

 8vo. Berlin, 1-832." 



