126 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[26. 



into two, and then ach of the two daughter-cells is similarly 

 divided into two ; this is characteristic of the development of the 

 microspores of Isoetes, and of the pollen-grains of most Monocoty- 

 ledons. 



2. The nucleus of the mother-cell divides into two, and both 

 these again divide into two before any cell-division occurs ; kino- 

 plasmic spindles are formed between the four nuclei (see Fig. 81) 

 in which cell-plates and eventually septa are formed simul- 

 taneously in the cell-plates, and thus the four special mother- 

 cells are formed, in each of which a spore is developed (p. 117). 



FIG. 81. Division into four of spore-mother-cells of Ferns. A Schizaea; B Pellcea 

 fotundifolia. I. II. III. successive stages; fc nucleus; p cell-plate; s spore; as' spore 

 lying behind the others; m wall of mother-cell; w septa formed in connexion with the 

 divisions (x 300). A Shows successive division ; B simultaneous division. 



The relative directions of the planes of division necessarily affect the arrange- 

 ment and form of the spores. Thus, when in (I) the divisions of the two 

 daughter-cells, or when in (2) the divisions of the two secondary nuclei, take 

 place in one plane, the four resulting spores lie in one plane, and have a rounded 

 form. Such spores are said to be bilateral ; they occur in the Cycads and 

 most Monocotyledons (pollen-grains), in Schizaea (Fig. 84 A III.), and some 

 other Ferns. On the other hand, when in (1) the divisions of the two daughter- 

 cells, or when in (2) the divisions of the two secondary nuclei take place in 

 two planes at right angles to each other, the four resulting spores do not lie 

 in one plane, but are arranged in a pyramid, each spore having the form of a 

 trilateral pyramid with a spherical base. Such spores are said to be tetrahedral, 

 or radial they occur generally in Dicotyledons (pollen-grains, Fig. 81), in some 

 Ferns (Fig. 84 B) and in Equisetum. In (2) when the four secondary nuclei all 

 lie in one plane four spindles are formed, each nucleus being thus connected 

 with the two adjacent nuclei ; but when the nuclei are arranged pyramidally, 

 six spindles are formed, each nucleus being thus connected with the three 

 others. 



In some plants the spores are developed sometimes in the one way and 

 sometimes in the other ; this is the case in the Liverworts and the Mosses ; 



