56 ANATOMY AND PHYSIOLOGY OF 



biuin, where the septa Ke parallel with the bark; in jointed hairs, 

 &c. When the secondary cells exhibit no more, or but very little 

 growth, this condition of the thickness of the walls is permanent, 

 and it is possible, when the membranes of the secondary cells 

 have been thickened by the deposition of layers, to distinguish 

 their membranes clearly, in their whole course, from the mem- 

 brane of the parent-cell, e,g.^ in the pith of Taxodium distichum. 

 On the other hand, when, as is usually the case, the secondary 

 cells increase much in si2:e after their production, this condition is 

 changed. In these cases the membrane of the parent-cell must 

 naturally share in the expansion of the secondary cells, and be- 

 come thinner in proportion to this expansion ; in consequence of 

 this the membrane of the parent-cell mostly vanishes completely 

 from the eye, especially when the division and with it the expan- 

 sion of the secondary ceUs is repeated. 



It has already been observed that the secondary cells completely 

 fill the cavity of the parent-cell at their first origin. Thus, no 

 traces of intercellular passages can be found in tissues in the first 

 stages of their development. The passages are formed subsequently 

 by the separation of the ceU-membranes at the angles of the cells, 

 and are not, as is usually represented, the remains of free open 

 spaces between globular cells which have been compressed together 

 in consequence of growth. In Hke manner the stomatal pores 

 are produced by the separation of two cells formed by the divi- 

 sion of a parent-celL 



Ohserv. That the formation of ceUti in all the organs of plants (ex- 

 cepting the cells originatiag in the embryo-sac) depends npon the division 

 of older cells, an opinion which could not, for a long time past, "be op- 

 posed by any carefol observer, unless he were misled by preconceived 

 notions. Even Meyen (" Physiologiej" ii. 334) declares this process of ceU- 

 formation to be very general; but linger {" Zinnma,'' 1841, 402; ^^ Bot 

 Zeit^' 1844, 489), who subsequently apphed to this process the term me- 

 rismatic cell-formation; andlsTageh (^' Zeitschr. f. wiss.Bot.^' iii. 49, 1846), 

 who used the expression pa/rietal cell-formation^ more especially asserted 

 the general occurrence of this process of formation ; the former declaring 

 to be the usual mode, the latter ascribing to it the production of all vege- 

 tative cells. 



But circumstances occurring iu the division of the cells were inter- 

 preted in a different way from what I have done. Meyen assumed that 

 the cell-membrane itself became folded inwards, and in this way formed the 

 partition, which is decidedly incorrect. linger thought the septum to be 

 origiaally simple, splitting afterwards into two lameUse ; NageH denied 

 tbat the septum is formed gradually from without inwards, assuming that 

 the membrane of the secondary cell is formed simultaneously all round its 

 cavity, whence it would of course result, that the septum composed of the 

 membranes of two contiguous secondary cells would be formed at once 

 across through the cavity of the parent-ceh. 



In reference to tMs latter point, I, of course, readily admit that one 

 seldom succeeds ia observing the gradual development of the septiun in 



