(HI 



TISSUES 



definite relation to the division of the nucleus, or at any rate to the 

 position taken lip by that organ; for the new cell-wall is laid down 

 midway between the two daughter-nuclei, while protoplasmic strands 

 and fibrils may often be seen to extend from the two nuclei towards the 

 advancing margin of the annular ridge. In the case of multi-nucleate 

 Thallophyte-cells, however, septation seems according to the data at 

 present available to take place quite independently of nuclear division. 

 The simultaneous segregation of numerous cells which takes place 

 in connection with the development of endosperm in the Angiospermous 



embryo-sac a process investi- 



MttoZ 





Fig. 8. 



gated more particularly by Stras- 

 burger is a modification of the 

 typical form of cell-division. In 

 the case of large and rapidly- 

 growing embryo-sacs the peri- 

 pheral protoplasm contains a very 

 large number (often several 

 thousands) of nuclei, all of which 

 owe their origin to the repeated 

 division of a single nucleus, the 

 so-called secondary nucleus of the 

 embryo-sac (after the fusion of 

 the latter with one of the genera- 

 tive cells of a pollen-tube). A 

 transitory cell-plate is formed 



Formation of endosperm in the embryo-sac of ill Connection with each of 



Agrimonia Eupatorium ; commencing development in i t 



of partition-walls between the nuclei. the tree HUCleai' CllVlSlOnS which 



After Strasburger (from Sachs, Lectures on the . . . 



PkysMogn of plants). give rise to these numerous 



nuclei ; this circumstance justifies 

 the inference that the mitoses in question were at some previous 

 stage of evolution accompanied by cell-divisions, a conclusion which 

 is further borne out by the fact that in small narrow embryo-sacs 

 the endosperm is produced by ordinary cell-division. When the 

 embryo-sac has finally completed its growth, but not before, each of 

 the numerous peripheral nuclei becomes connected with all its neigh- 

 bours by groups of spindle-fibres, within which cell-plates now once more 

 appear (Fig. 8) ; but on this occasion the cell-plates give rise to septa 

 which form a mesh work of cell- walls projecting inwards from the wall 

 of the embryo-sac. The endosperm cells produced in this way are thus 

 at first open on their inner side ; ultimately, however, they become 

 shut off by cell-walls from the liquid contents of the embryo-sac. 

 During the further development of the endosperm, its cells multiply 

 solely by ordinary division. 



