236 THE PHENOMENON OF 



where Mohl describes the formation of the cells in young 

 points of stems and roots, and in the cambium layer of 

 Dicotyledons, he expresses a conjecture that the division 

 of the cells he observed in the Confervas is a totally 

 different process from the multiplication of cells in the 

 higher plants, since in cells dividing in this way no 

 nucleus can be found, while in the cell-formation of the 

 Phanerogamia these play an essential part. In the 

 multiplication of the cells of the Phanerogamia, moreover, 

 he never could observe a gradual constriction of the pri- 

 mordial utricle. In seeking to advance this difficult 

 point nearer to its solution, I remark, first of all, that 

 the essential part of the problem is not so much the 

 question whether the cellulose septum is formed in 

 gradual progression towards the centre, or simultaneously 

 over its entire surface, but the question, whether the cell- 

 contents are divided by gradual annular constriction 

 (with in-folding of the primordial utricle), or through 

 simultaneous separation in the place of the entire sectional 

 plane, and the formation of two new coherent primordial 

 utricles. The succeeding formation of the septum might 

 be simultaneous in its entire extent, even in the first 

 case, if, namely, the formation of membrane on the whole 

 surface of the primordial utricle, did not take place until 

 the division was complete; and, on the contrary, the 

 formation of the septum might advance in annular incre- 

 ments, in the second case, if the secretion of membrane 

 on the newly-formed plate of the primordial utricle, did 

 not take place over the whole surface simultaneously, but 

 at the periphery earlier than in the centre. In the first 

 place, it is certain, notwithstanding Nageli's objections, 

 that the description given by Mohl of the cell-division 

 taking place in Cladophora, by gradual constriction of 

 the contents, corresponds to the truth. I have seen this 

 process so completely and clearly in the large species of 



Zeit, 1844, pp. 289, 291,) (Trausl. in ' Taylor's Scientific Memoirs,' vol. iv, 

 p. 96. A. H.) 



