108 



PHYSIOLOGICAL VIEWS. 



branous substance appears chiefly under two elementary forma. 

 viz. 1st, that of cellular texture; 2d, vascular texture. 



1st. Cellular texture, (Fig. 

 116, a;) this, according 

 to the opinion of Mirbel, is 

 composed of a mass of little 

 hexagonal cells, resembling 

 honey-comb. Another wri- 

 ter* compares the appearance 

 of the cellular texture to the 

 froth of fermenting liquor : 

 he considers that each cell is 

 116. IlllJflii disconnected with the others; 

 while Mirbel believes that the 

 divisions of the membrane, which forms these cells, are common to 

 contiguous cells. The cellular system in animals contains the fat; 

 in vegetables it is generally filled with resinous, oily, or saccharine 

 juices ; in some cases the cells contain air only. They are usually 

 marked by small dots, (as at a. Fig. 116;) these are supposed to be 

 apertures, through which fluids are transmitted from one cell to 

 another. 



The cellular texture composes most of the pith, parenchyma, and 

 cotyledons of almost all vegetables. It is abundant in tuberous roots, 

 pulpy and fleshy fruits, and the stems of grasses, and constitutes the 

 principal part of mushrooms, and other cryptogamous plants. In the 

 bark of plants the cellular texture is situated under the cuticle ; it is 

 filled with a juice which varies in colour in different species of plants, 

 but is most commonly green ; it gives its colour to the bark, as the 

 same texture under the human cuticle gives colour to the skin. The 

 green colour of leaves is caused by the cellular texture, which is en- 

 closed on both sides by the cuticle. In the pith of young plants, the 

 cells are filled with watery fluids, but in older plants they are empty, 

 or only filled with air. The petals of flowers owe their beautiful 

 hues to the presence of cellular texture, filled with juices, which 

 refract and reflect the rays of light, in a peculiar manner. 



Vascular] texture, consists of tubes, which, like the vessels of the 

 animal frame, are capable of transmitting fluids. These tubes are 

 open at both ends, and are protected by a coating of cellular in- 

 tegument ; their sides are thick and almost opaque. These vessels 

 extend throughout the whole plant, distributing air and other fluids 

 necessary to Vegetation. The vascular system of plants presents a 

 variety in form, and also with respect to the functions which the 

 different parts perform. 



Some are entire vessels, or without any perforation, (Fig. 116, c ;) 

 these convey the proper juices of the plant, and generally contain 

 oils and resinous juices. 



Porous vessels exhibit many perforations, (Fig. 116, b ;) they often 

 separate and again unite, changing at length into cellular integu- 

 ment. 



* Dutrochet. 



t The term vascular i 



derived from the Latin woxAvasculum, a little vessel. 



Two iiind of membranes — Cellular texture— how situated — Cause of the green co 

 lour of plants— of the hues of petals— What does the vascular texture consist ofl— 

 Entire vessels— Porous vessels. 



