26.] 



CHAPTER I. THE CELL. 



127 



amongst Ferns ; whereas in the Hymenophyllaceae and Cyatheaceae only tetra- 

 hedral spores have been observed, the occurrence of tetrahedral or of bilateral 

 spores varies from genus to genus in tbe Polypodiaceas, Schizeacese, and 

 Gleicheniaceae ; in the Marattiaceas and in Ophioglossum both tetrahedral and 

 bilateral spores may be developed in the Fame sporangium ; this last condition 

 also obtains in Psilotum and in species of Lycopodium ; both modes of 

 development have been observed in the pollen-grains of Allium Moly among 

 Monocotyledons and also in some Dicotyledons (Fig. 85). 



The division of the cytoplasm of the spore mother-cells is fre- 

 quently accompanied by a considerable ingrowth of the wall of 

 the cell, so that the cytoplasm becomes lobed (Fig. 85) ; the septa 

 subsequently formed in the cell-plates become attached to these 

 ingrowths. In some cases of cell-division the whole septum is 

 formed as an annular in- 

 growth (e.g. Spirogyra). 

 This is also the case in in- 

 completely septate plants 

 (e.g. Cladophora, etc.), 

 where the development of 

 the septa has no relation 

 whatever to nuclear divi- 

 sion. 



In incompletely septate 

 plants the process of pro- 

 toplasmic division and the 

 development of septa goes 

 on quite independently of 

 the nucleus. This is so 

 to some extent in certain 

 cases of spore-formation, 

 viz. in the development of 

 the spores of Anthoceros 

 and of the macrospores of Isoetes, where the division begins in 

 the cytoplasm, the division of the nucleus taking place last of all ; 

 whereas, as has been already shown, the rule in completely septate 

 plants is that the division of the nucleus precedes the division of 

 the cytoplasm. 



FIG. 85. (After Sachs). Development of pollen- 

 grains of Althaea rosea. A the four nuclei have 

 been formed ; D the cytoplasm is becoming lobed 

 by the ingrowths formed on the wall of the mother- 

 cell ; E four special mother- cells formed by the 

 development of septa connected with the lateral 

 ingrowths. In A D E the grains are developed in 

 one plane ; B is a case of pyramidal arrangement. 



