FORMATION OF CELLS. 



13 



parts, either at once or by a repeated bipartition. In the special details of cell-division, 

 however, some deviations occur. 



[The first stage is, in all cases, the division of the nucleus of the mother-cell into 

 two. This may be at once followed by the simultaneous formation of a cell-wall in 

 the plane of the cell-plate (see infra, The Behaviour of the Nucleus during Division), 

 as in the development oPthe pollen-grains of most Monocotyledons, and of the micro- 

 spores of Isoetes ; a repetition of these processes in each of the two daughter-cells leads 

 to the formation of the four pollen-grains or spores. More commonly the cell-plate 

 between the two nuclei undergoes absorption ; the two nuclei then divide, and the 

 missing cell-plates are reconstituted ; cellulose walls are now simultaneously formed in 

 the cell-plates so as to divide the protoplasm into four parts, each containing a nucleus, 

 which constitute the spores or the pollen-grains. 



The division of the two secondary nuclei takes place in certain cases (pollen-grains 

 of most Monocotyledons) in one plane, and this plane is at right angles to that of the 

 division of the nucleus of the mother-cell. As the result of this, the four spores or 

 pollen-grains formed lie in one plane and have a rounded form ; they are said to be 

 bilateral. In other cases (pollen-grains of Dicotyledons, spores of Equisetum) the division 

 of each secondary nucleus takes place in a plane which is at right angles to that of the 

 other and to that of the nucleus of the mother-cell; as a consequence the four spores 

 or pollen-grains do not lie in one plane but are arranged tetrahedrally, and have more- 

 over a somewhat tetrahedral form; they are said to be radial. In some plants the 

 spores or pollen-grains are formed sometimes in one way and sometimes in the other, 

 and are therefore either bilateral or radial. This is the case, for instance, in Liverworts 

 and Mosses: amongst Ferns, whereas in the Hymenophyllaceae and Cyatheaceae only 

 radial spores have as yet been observed, radial spores have been found in some and 

 bilateral spores in others of the genera of Polypodiaceae, and this is probably also the 

 case in the Schizaeaceae and Gleicheniaceae : in the Marattiaceae and in Ophioglossum 

 radial and bilateral spores may be produced in the same sporangium : this last condition 

 also obtains in Psilotum and in Lycopodium Selago and inundatum : finally, these two 

 modes of the development of the pollen-grains have been observed by Strasburger in 

 Allium Moly among Monocotyledons. 



In the following paragraphs a detailed account is given of the course of development 

 in certain cases illustrating the two modes above-mentioned. 



a. No cell-nvall is produced until after four nuclei ha've been formed by division ; the 

 resulting cells are arranged tetrahedrally.^ 



Development of the spores of Equisetum. At first the mother-cells swim in the fluid 

 which fills the cavity of the sporangium, in groups of two or four together (Fig. 10, 

 a, b). Each mother- cell consists at first of a large spherical nucleus (including nu- 

 cleoli), surrounded by fine-grained protoplasm, with a sharply-defined outline, but is 

 without a cell-wall ^. [The first indication of division is the coalescence of the granules 

 in the nucleus to form fibrillae ; it becomes elongated in form, assuming a spindle-shape. 

 An aggregation of coarse granules now makes its appearance in the equatorial plane 

 of the spindle, constituting the nuclear disc : this splits into two discoid halves, and 

 each half then travels to one pole of the nucleus and there forms a new nucleus. 

 A fresh aggregation of granules now appears in the equatorial plane of the spindle, 

 constituting the cell-plate ; this extends quite across the protoplasm of the cell. The 

 fibrillae connecting the two new nuclei with the cell-plate now disappear, and then the 

 cell-plate also. During this time the two nuclei have begun to divide in the same way 

 as the primary nucleus, in planes at right angles to each other and to that of the 

 division of the primary nucleus. Six cell-plates are now formed between the four 

 nuclei, and become connected with them by the formation of fibrillae. In the cell- 

 plates cellulose walls are now formed and the division is complete. The protoplasm 

 of each of the young spores rounds itself off, and its nucleus assumes a central position 



' [According to Strasburger (loc. cit.) a delicate cell-wall is present,] 



