H 



MORPHOLOGY OF THE CELL. 



and a rounded form. The next step is the absorption of the cellulose walls and the 

 setting free of the young spores as primordial cells, which soon become clothed with 

 a cell-wall. This cell-wall subsequently becomes differentiated in a manner which is 

 described in detail on page 403. 



In other cases the wall of the mother-cell presents ingrowths at points correspond- 

 ing to the lines along which the cellulose walls are formed, and this conveys the erroneous 

 impression that the wall of the mother-cell grows inwards constricting the protoplasm 

 and ultimately dividing it into four parts. A good example of this case is afforded by 

 the development of the pollen-grains of Dicotyledons. Fig. 11, taken from Tropxolum 

 minus, will serve to illustrate it. In a four nuclei are present, which have been formed 

 in the manner described above with reference to Equisetum : they are connected by 



y 



Fig. ri.— Mode of development of the pollen of Tro- 

 pcBolu)n ntiHus (X 550 and reduced). 



Fig. 10 — Mode of development of the spores of Equisetum liniosttm (X 550) ; a group of four, b group of two^ mother-cells ; 

 c and (i mother-cells preparing for division, showing the nuclear disc ; e one with two nuclei ; y, g, and i division into four spores ; 

 h abnormal formation of three spores from one mother-cell. 



fibrillas, and six cell- plates are present in the equatorial planes of the groups of fibrillae 

 which connect the nuclei. The wall of the mother-cell is thickened, especially at two 

 parts of its surface, but it presents no ingrowths ; but now {b) ingrowths begin to be 

 formed at points which correspond to the superficial ends of the six cell-plates {f,g,h,k'). 

 Their growth does not, however, proceed so as to cause the constriction of the proto- 

 plasm into four masses : it is soon arrested ; the cell-walls are formed simultaneously 

 in the six cell-plates and are attached externally to the ingrowths of the cell-wall of 

 the mother-cell. Later each mass of protoplasm forms a new proper wall around 

 itself, which is the wall of the pollen-grain ; the thick cell-vi^alls surrounding them are 

 absorbed, and the four pollen-grains are set free. (See also p. 551.) 



b. A cell-'^all is formed after each nuclear di'vision ; the resulting cells may be arranged 

 either tetrahedrally or in one plane. 



This mode of cell-division closely resembles that which takes place in a growing 

 tissue. It may be very clearly observed in the formation of the pollen-grains of most 

 Monocotyledons. Fig. 12 shows the process in Funkia ovata. In / two nuclei are 



