Development, and Structure of the Vegetable Cell. 279 



It would appear that those materials which are subservient to the 

 growth of the young cell-wall pass from the lower to the upper 

 portions of the plant, whilst chlorophyll is produced from com- 

 pounds which diffuse themselves from the upper into the lower 

 parts of the plant. 



During the first period, which ensues upon the unfolding of 

 the annular fold and the extension of the cells, the upper young 

 joint-cell usually remains more or less unchanged; the lower 

 one, on the contrary, continues to enlarge, whereby the new and 

 still delicate transverse septum becomes pressed upwards, until 

 it at length projects from the opening in the lacerated mem- 

 branous sheath, and then it proceeds to increase in thickness. 

 (PI. V. figs. 23, 34.) 



Bary asserts that he distinctly made out that the septum is pro- 

 duced by a gradual constriction and secretion of the primordial 

 layer, in various species of (Edogonium, after the protrusion of 

 the clear lamina from the membranous sheath. In the treatise 

 quoted (at p. 42) he says : — " In any case, the septum is not 

 simultaneously formed in its entire surface ; on the addition of 

 a solution of chloride of zinc and iodine, the contracted primor- 

 dial layer is sometimes distinctly seen to pass through the 

 middle of an incompletely closed dissepiment." 



That this phenomenon, which I have figured in a Spirogyra 

 (PI. VII. fig. 67), and to which I shall hereafter revert in con- 

 nexion with Cladophora, furnishes no sufficient proof of the pro- 

 duction of 'the dissepiment by constriction, will be shown when 

 I come to speak of the latter genus. 



Of the formation of a fold in the membrane of the mother 

 cell, after the two daughter cells (figs. 21, 26, 28) have become 

 mutually pressed together so as to form a complete septum 

 • (figs. 22, 27, 29), and of an eventual inward extension of this fold 

 betwixt the lamellae of the septum, which are thus again sepa- 

 rated, I have no knowledge ; but the more distinct appearance 

 of the long-previously existing septum can, in my opinion, be 

 much more readily accounted for by the gradual thickening that 

 progresses in it from its circumference. That this thickening 

 of the septum always follows after its emergence from the enve- 

 loping sheath is in all probability to be attributed to the changes 

 in the nature and operation of the nutritive matters derived from 

 without, now only through the walls of one assimilating and 

 secreting system of cells. 



In the mean while, the upper of the twin cells also begins to 

 grow vigorously, until at last it equals the lower one in length. 

 Nevertheless it not unfrequently remains somewhat shorter, 

 which is the cause of the irregularity seen in the structure of 

 (Edogonium. 



