DEPENDENCE OF DIVISION UPON THE FORM OF THE CELL. 



433 



the individual cells of which grow independendy after each division ; as in the 

 genera Chlorococcus, Merismopedia, Tetraspora, Glceocapsa, and many others. In 

 more highly developed plants similar processes take place in the mother-cells of 

 spores and pollen grains. In these cases the fact is particularly clear, that the 

 mode in which the cell-divisions follow one another depends by no means on the 

 physiological or morphological nature of the cells, but upon their mode of growth 

 and external form — especially upon the latter. Fig. 267, for example, shows six 

 different forms of cell-division in the pollen mother-cells of one and the same 

 plant, the Orchid Neottia nidus-avis. In A the pollen ffiother-cell had approximately 

 the form of a circular disc, which, by means of two divisions at right angles to 

 one another, is cut up into four quadrants. It is to be noticed, however, that the 

 two vertical division-walls in Fig. A do not exactly meet one another, so that 

 a small piece of the horizontal division-wall remains intercalated between the points 

 where they join it : this intercalated piece of the previous division-wall appears like 

 a breaking of it, and we meet with such interruptions of the walls very generally 

 in the division of tissue-cells, a point on 

 which I lay some stress, because in the 

 more complex cases of tissue-formation 

 they interfere with the otherwise easily 

 recognisable regularity of the network of 

 cell-walls. 



The pollen mother-cell C had grown 

 more vigorously in one direction, and had 

 become, long and .elliptical before its divi- 

 sions. ■ Accordingly, in the first place two 

 transverse walls were formed at right 

 angles to the long axis of the cell, so that 

 they divided the latter into three approxi- 

 mately equal parts : the middle one of these 

 cells has again been divided, however, by a 

 longitudinal wall. Here again, therefore, 



four daughter-cells have been produced by division of the pollen mother-cell, but, as 

 is seen, in an order different from that in A, corresponding to the elongated form 

 which the mother-cell had assumed before the division. Whether the form of 

 division represented in Fig. B was developed from the type A 01 C cannot well 

 be decided ; but in £ as well as in C we perceive how the primitively straight or 

 slightly curved walls appear broken after the joining on of a younger wall, somewhat 

 like stretched threads strung together by another thread. The division-walls which 

 meet one another correspond with their three angles to a so-called string-polygon, as 

 also is usually the case in a multicellular tissue. 



The dependence of the directions of division upon the external form of the 

 mother-cell is manifested again in another way in Fig. D. Here the mother-cell 

 consisted of a sort of cylindrical stalk with a little spherical head : accordingly, two 

 walls have arisen in the stalk-like portion cutting the external wall at right angles ; 

 in the hemispherical head, however, the wall is longitudinal. In Fig. E] the mother; 

 cell was approximately globular before the division : it is bisected into two hemi- 



[3] Ff 



- Fig. 267.— Differently shaped and therefore differently 

 divided pollen mother-cells from one and the same anther of 

 Neottia nidus'avis (after Goebel). 



