163 



On the Histology of Plants. 



By R. Braithwaite, M.D., F.L.S. 



II. Origin of the Plant Cell. 



{Read June 27, 1873.) 



Haying become acquainted witli the structure of the cell as an 

 individual, we have next to consider how cells originate, for on the 

 constant formation of new cells all growth depends. 



A plant cell always arises within another, which is thus ap- 

 propriately termed the mother-cell, and it is either the contents 

 or the primordial membrane of the mother-cell, which is the active 

 agent in the process ; in the former case we have Free cell forma- 

 tion^ in the latter Cell formation by division. Dippel has so ably 

 elucidated this difficult subject that I cannot do better than lay 

 before you the result of his observations throughout. 



1. Free Cell Formation. — This was first noticed by Wolf a 

 century ago, but for a clear definition of the process we are in- 

 debted to the researches of Prof. Schleiden. It is of universal oc- 

 currence when once the first cell of a commencing organism has 

 been formed, and may be observed in the spermatozoids and spore- 

 cells of the higher cryptogams, in all germ cells, in pollen cells, 

 and in the embryo sac of Liliaceae, Iridace^e, Compositte, Onagracese, 

 &c. 



Two modifications may be distinguished — (1) when one or more 

 free cells arise without the original contents being consumed ; (2) 

 when one or more daughter-cells use up the contents of the mother 

 cell, which thus perishes, its cellulose case continuing for some 

 time, protecting the new cells. The first form is seen in the em- 

 bryo sac of flowering plants, and may be readily observed in the 

 Liliacese, where this organ is very large, even just after impregna- 

 tion. A thin central slice is placed in very weak solution of gum, 

 and in the parietal protoplasm of the embyro sac we may find all 

 stages of the growing cell containing free nuclei of various sizes. 



