224 PHYSIOLOGICAL BOTANY. 



gratefully acknowledge that Mohl has now first taught 

 us the true structure of the membrane of the cell ; that 

 the wall of the cells and vessels is composed of a primary, 

 external, imperforate membrane, and a secondary one 

 which is usually perforated with apertures. It constitutes 

 the basis of our knowledge upon this subject. We may 

 add, with Payen, the outer membrane is not coloured 

 yellow by iodine, whilst the internal lining is so. Mohl 

 further adds, that the internal membrane consists of 

 superimposed layers. This is by no means rare, espe- 

 cially in the solid, cartilaginous, so-called stony cells, 

 several remarkable examples of which have been adduced 

 by the author also in this memoir ; but is not found in 

 all, at least has not been positively detected. Why, then, 

 should we admit their existence in parts where they are 

 not visible? How the primordial utricle is converted 

 into a cell having a separate existence, the author does 

 not by any means show; and we shall hereafter allude 

 to the fact, that it not only exists in the young cells, but 

 also in those which have completed their growth, and not 

 unfrequently even in old cells, provided they have not 

 become too solid and cartilaginous. But when Mohl 

 says, that the increase of cells takes place either by the 

 division of the older cells, by means of a newly -formed 

 septum, or by the formation of cells within other cells, a 

 third plan is evidently overlooked viz. the formation of 

 new cells between the old ones. Mirbel has already 

 shown this in his memoir upon Marchantia. It appears 

 to me to be the true manner in which their increase 

 takes place. I have had the anatomy of the bulb of 

 Amaryllis formosissima drawn in my Plates (Part I, 

 pi. 1). We there see, in fig. 4, at the base of the leaves, 

 a zone of short, laterally distended cells, with thinner 

 walls than those above and below them. Hence they 

 appear to have been recently formed ; moreover, the gra- 

 nules contained within them are not coloured blue by 

 iodine, as the granules in the cells, which are situated 



