CELL-DIVISIONS INDEPENDENT OF THE NATURE OF THE ORGAN. 



435 



the configuration of a growing organ, and not ujjon its morphological or physio- 

 logical significance, is one of the most general and important facts to which I have 

 referred in my treatises already quoted. It was formerly supposed to be possible 

 to characterise the true morphological or phylogenetic nature of an organ by the 

 way in which its cell-divisions took place, and hundreds of treatises and laboriously 

 drawn plates were devoted to this purpose. In the exposition which follows, I shall 

 have plenty of opportunities of showing how erroneous this assumption is. Mean- 

 while, however, I will refer to a particularly clear case (Fig. 270). It is at once 

 obvious that Figs. C and D exhibit essentially the same laws of cell-division : in both 

 cases we have a stalked capitulum, with transverse divisions in the stalk, and in the 

 head itself transverse and longitudinal divisions which cut one another at right 

 angles. But C is a glandular hair of the 

 Gourd plant, and D a very common form 

 of the embryo of a Phanerogam. In Fig. 

 A also we at once recognise the type 

 of cell-division represented in C and D, in 

 accordance with the outline of the simple 

 organ ; but here the stalk remains undi- 

 vided, having only formed the cell marked 

 /i {Hypophysis). Above this the capitu- 

 lum becomes divided into sectors, which, 

 as shown in B, grow in breadth, and so 

 give to the whole glandular hair the form 

 of a mushroom. The comparison of the 

 most various organs of the most different 

 nature would only afford further examples 

 of what has been stated above — that the 

 form of the cell-division depends entirely 

 upon the growth and outward form (espe- 

 cially the latter), and not upon its physio- 

 logical and morphological significance. 



Among the most instructive relations 

 between cell-division and growth I count 

 also the fact that the cell-division need not 

 take place during growth itself, but may 



appear only after its conclusion. In this connection, a particularly clear case has 

 been established by Geyler in the shoots of Slypocaulon, one of the Algae belonging 

 to the Phseosporeae (Fig. 271). The whole of the apical region of such a shoot, 

 marked s and z, as well as the region of the lateral shoots marked x and y, 

 corresponds not only to the growing-point of an ordinary cellular shoot, but also to 

 the portion which is elongating. Only the parts which have grown to their full 

 extent, and marked I, II, etc., become gradually converted by division-walls into 

 smaller and smaller cells, as shown in Fig. 271 ; and Geyler has established that in 

 the course of these cell-divisions no more growth takes place. We have therefore, 

 in this case, at the upper end of the shoot growth without cell-divisions, and in the 

 older portions of the shoot cell-divisions without growth. 



F f 2 



Fig. 270.-^4, B hairs of different ages from the leaf of 

 Piiigtticula VKtgaris ; C hair of Cucurbita ; D embryo of Nico- 

 tiana (tobacco) ; h the so-called Hypophysis. 



