PHYSIOLOGICAL BOTANY. 223 



seen just as beautifully as by the long preservation in 

 spirit. As the primordial utricle exists in all young cells, 

 the author thinks that it contributes to the formation and 

 growth of the cells ; for, he adds, we can only conceive 

 cell-growth to occur in two ways, either by division of 

 the older cells by the formation of a septum, or the forma- 

 tion of cells within other cells. He believes that in the 

 cambium-layer of Pinus sylvestris, Sambucus ebulus, As- 

 depicts Syriaca, and Euphorbia Caput Medusa, he has 

 seen two primordial utricles prior to the appearance of a 

 septum between them, which, therefore, confirms the 

 latter method of formation. However, he is by no means 

 free from doubt on this point. What we have just 

 asserted entirely agrees with Schleiden' s theory, except 

 that Schleiden believes the cell-membrane to be formed 

 from the nucleus. Mohl, on the other hand, considers 

 that the cell-membrane always surrounds the nucleus. 

 Moreover, according to Schleiden, the cell-membrane first 

 formed constitutes also the later one the external mem- 

 brane of the cell; whilst, according to Mohl, the mem- 

 brane of the primordial utricle becomes the external 

 membrane. Hermann Karsten mentions having seen 

 the primordial utricle in his memoir, ' De celle Vitali ;' but 

 he confounded it with the internal layers of the membrane 

 of the cell. The author names several excellent exam- 

 ples of the various cell-membranes, and concludes, in 

 opposition to Hartig's opinion, as follows : " The above 

 remarks show that a positive decision of the question, 

 whether the cells are invested with a special membrane 

 or not, is accompanied with no little difficulty, since 

 optical illusion (Mohl means a luminous appearance) and 

 a slight modification in the substance of the innermost 

 layer of the cell, and this may also occur in the inter- 

 mediate layers, may readily lead to the belief that such a 

 membrane has been found. Hartig obtained his proof 

 from the cells of Taxus baccata, in which Mohl long 

 since showed that there existed a third layer. We must 



