DR. M. J. SCHLEIDEN OX PHYTOGENESIS. 309 



vessels of the ^A•oocl" is something immensely different from the 

 spiral vessels of herbaceous plants, both with respect to their 

 origin and probably also with respect to their physiological 

 destination. In the controversies carried on, sometimes with 

 great warmth, respecting the function of spiral vessels, no 

 result has been obtained, nor could any be obtained, because 

 each person meant, quite ad libitum, the spiral vessels of hei'- 

 baceous plants, or of the wood, completely shutting their eyes 

 to the possibility that the two might be exceedingly difFei'ent 

 things. If, for instance, we consider the cambium in the ear- 

 liest period in which it begins to acquire organization, we find 

 that it consists throughout of entirely similar prosenchymatous 

 cells still in a gelatinous state. A short time afterwards some 

 longitudinal series of these cells appear to have increased in 

 breadth, by Avhich alone they are distinguishable from the adja- 

 cent mass. On a further development we observe that some 

 dark spots appear on the walls of some of these expanded cells, 

 which we soon recognise to be small flat air-bubbles that have 

 formed between the walls of tliis and of the neighboiu-ing cell. 

 Gradually all the expanded cells which are superposed one upon 

 the other are changed in this way ; the air-bubble gradually ap- 

 pears more circularly or ovately bounded, and there appears in 

 its centre a smaller circle which, constantly becomes more dis- 

 tinct, and which originates in the following manner : — on the 

 deposition of new masses upon the inner wall of the cell, the 

 parts corresponding to the outer air-bubble remain free from 

 this deposition, thus forming a small canal which traverses the 

 newly deposited mass. We now distinguish the fully developed 

 porousvesseljthe septa between each two superposed cells appear- 

 ing at the same time to be more or less reabsorbed. This history 

 of the formation of the porous vessels, which may easily be ob- 

 served on limes and willows, greatly contradicts the general no- 

 tion that the porous canals serve to facilitate the communication 

 of the saps. As the air-bubble is first formed on the outer sur- 

 face of the wall, it renders the passage of the sap impossible at 

 this spot, and for this reason the oi'igin of the porous canal 

 might probably be most easily and naturally explained as a lo- 

 cal atrophy of the cellular wall. At the same time it is evident 

 from hence that the distinction between wood in general \Iaub- 

 holz] and fir-wood, as to its anatomical structure, cannot be of 

 such vast physiological importance ; for, with Uke elements and 



VOL. II. PART VI. X 



