211 



On the Histology of Plants. 



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



III. Transformation of Cells. 



(Read Nov. 28th, 1873.; 



We pass now from individual cells, and proceed to study the 

 changes produced in tliem by growth, and by combination into 

 tissues. In the young state, nutrition is active, and the elements of 

 the plasma contained in the cells are capable of rapid assimilation, 

 consequently we have an increase in the whole volume of the cell, 

 both the primordial membrane and cellulose case participating, and 

 then commences an internal deposit of cellulose, by which the cellu- 

 lose case becomes thickened to a greater or less extent. This 

 increase in volume we may assume takes place in three directions 

 — vertically, antero-posteriorly, and laterally — so that a change in 

 form is soon set up in the growing cell, and this in two ways ; firstly, 

 nutrition may not proceed uniformly in all parts of the cell, and thus 

 extension takes place in certain directions, and not at all in others ; 

 secondly, the rapidly enlarging cells soon begin to press against 

 each other, and thus become moulded into various shapes. So long- 

 as the cells remain free, and nutrition progresses equally on all 

 sides, they retain their sj^herical form, as we may observe in the 

 spores of Alg^e and mosses, in pollen, and in the soft pulp of fruits ; 

 but if they combine into a tissue, their spheroidal shape is changed 

 by mutual pressure, and they acquire a more or less polyhedral 

 form, as we may observe in the albumen of seeds, and in pith cells, 

 or in the potato. 



If nutrition proceeds in all three directions, but not equally over 

 the whole surface, we have radiate forms, as in stellate hairs, or if 

 it be still more irregular we get branched cells, such as we see in 

 the hairs in the air passages of NympTicea or in the bast-tissue of 

 Conifers ; if nutrition proceeds only in two directions, and is arrested 

 JouRN. Q. M. C. No. 25. o 



