THE VEGETABLE CELL. 57 



consequence of the folding inward of tlie primordial utiicle, but in par- 

 ticular cases I have seen this process most distinctly. The description 

 giyen above rests chiefly on observations which I instituted npon the 

 parent-cells of pollen-grains, and in the cells which separate from each 

 other in the pore-cells of stomates. Mirbel (" Eecherches bur U MarcJmn- 

 tia") detected, in 1833, that the parent-cells of the pollen-grains divide 

 by septa which grow from without inward ; but the correctness of this 

 statement was denied by Nageli {'' Untwickelungsgesch. d. Pollens^' 1849), 

 who asserted that secondary cells (which he called special parent-cells) 

 were formed in the interior of the parent-cells, and that the seeming septa 

 were nothing else than the coherent walls of these cells, which were not 

 formed in the direction from without inwards, but simultaneously all over 

 the contents,— a view which was shared also by Hofmeister (" Entw. d. 

 Polleoisr — Bot Zeit. 1848, 654). That these representations are incorrect, 

 and that the septa grow from without inwards {see pi. 1, fig. 8 — 11, 

 which represent different stages of development of the parent-cells of the 

 pollen-grain of Althoea rosea), was akeady stated by linger (" Ueb meris- 

 matisch, Zellenbild^mg hei der Entwick. der Follenhorperr — " Bericht. der 

 Ver. der NaturforscK zu Gratz.^'), and no doubt remained in my mind, 

 since I succeeded in bursting parent-cells of pollen-grains, the septa of 

 which were but half-formed, and pressing free the primoidial utricle (pi. 1, 

 ^g. 10), which was half constricted by folds passing inwards, into foxir 

 globular subdivisions connected together into a common cavity in the 

 centre. I have elsewhere (" Veron. Schrift,^^ 2d2) sought to demonstrate 

 that in like manner in the formation of stomates, there is no production 

 of secondary cells in the parent-cell, with an intercellular space runnhig 

 between them, as Nageli states. The observations of Henfrey (" Annals 

 of IF at. History y' vol. xviii, 364) are in exact accordance with mine. Of 

 course one does not succeed in the vast majority of cases of the examina- 

 tion of a tissue where the celL are in course of development, in observing 

 the gradual growth of the septa from without inwards, and when I as- 

 sume that this process occurs universally, I certainly rest upon the analogy 

 to the fe^ case^ m wMch I have traced their gradual de^lopmeat , bft 

 it seems to me more logically correct to lay the main stress upon a few 

 accurately investigated cases, than to disregard such observations, and to 

 use as the basis of the theory of the development of cells, the imperfect, 

 though numerous, observations in which the gradual growing in of the 

 septum was not seen, but the mode in which it really was formed was not 

 perceived at all. 



h. Free Cell-formation* 



In free cell-formation, the cell-membrane is developed over the 

 surface of a mass of nitrogenous substance swimming in a fluid 

 which contains formative matter, without the co-operation of a 

 parent-cell. In the regular course of vegetation this process of 

 cell-formation occurs only in the interior of cells ; it may occur 

 independently of the life of the parent plant in the creation of 

 parasitic Fungi, Yeast cells, &c., both in the decomposing fluid of 

 cells and in the excreted or expressed juices. In normal free cell- 

 formation the secondary cells usually possess but a very small size 



