238 



On the Histology of Plants. 



By R, Braithwaite, M.D., F.L.S., &c.' 



IV. The Different Kinds of Cells. 



{Read March 27th, 1874.) 



Having followed the vegetable cells from the homogeneous 

 spherical condition through various stages of differentiation, we 

 have next to consider them in their finished or perfect form. 

 Many names have been introduced — especially by Nageli — for the 

 various kinds of cells, but often unnecessarily, as for instance 

 Sclerenchpna, when they have become thickened and indurated; 

 but as all cells may undergo this physiological change it is clear 

 it possesses no distinctive character, for it applies to wood, to the 

 stony concretions of the pear, to the dark bundle in the rhizome 

 of the brake, and many other tissues. 



We may more simply classify them by the structure of the 

 cellulose case, and their mode of connection with each other, and 

 thus they readily fall into three groups — Parenchym cells, Fibre 

 cells, and Tubular cells, which we may not inaptly compare to the 

 flesh, bones, and vessels of animal bodies. 



I. — Parenchym Cells. 



These include all spherical, polyhedral, or prismatic cells, which 

 are applied to each other by their cellulose case, and having their 

 walls homogeneous or variously thickened, and only rarely pro- 

 vided with bordered or sieve-like pores. The term Tarenchyma 

 has often been restricted to those cells which have flat ends, while 

 those with pointed extremities have been named Prosenchyma. 



Parenchymatous cells constitute the entire substance of Fungi, 

 Lichens, Algae, and the greater part of mosses ; in the higher 

 plants all homogeneous tissues, as pith, bark, and cuticle, and in 

 the heterogeneous tissues they are combined with the other two 

 forms in all vascular plants. Thin walled Parenchym cells are of 

 most frequent occurrence, and assume many different forms. The 



