STRUCTURE.] FORMS OF CELLULAR TISSUE. 49 



MEMBRANOUS CELLULAR TISSUE is that in which the 

 sides consist of membrane only, without any trace of fibre ; 

 it is the most common, and was, till lately, supposed to be 

 the only kind that exists. This sort of tissue is to be consi- 

 dered the basis of vegetable structure., and the only form 

 indispensable to a plant. Many plants consist of nothing 

 else ; and in no case is it ever absent. It constitutes the 

 whole of Mosses, Algals, and Lichens ; it forms all the pulpy 

 parts, the parenchym of leaves, the pith, medullary rays, 

 and principal part of the bark in the stem of Exogens, the 

 soft substance of the stem of Endogens, the delicate mem- 

 branes of flowers and their appendages, and both the hard 

 and soft parts of fruits and seeds. 



It appears that the spheroid is the figure which should be 

 considered normal or typical in this kind of tissue ; for that 

 is the form in which bladders are always found when they 

 are generated separately, without exercising any pressure 

 upon each other ; as, for example, is visible in the leaf of the 

 white lily, and in the pulp of the strawberry or of other soft 

 fruits, or in the dry berry of the Jujube. All other forms 

 are considered to be caused by the compression or extension 

 of such spheroids, or by the action of organisable matter in 

 unequal degree. 



When a mass of spheroidal bladders is pressed together 

 equally in all directions, rhomboidal dodecahedrons are pro- 

 duced, which, if cut across, exhibit the appearance of hexa- 

 gons. (Plate I. fig. 12.) This is the state in which the 

 tissue is found in the pith of all plants ; and the rice paper, 

 sold in the shops for making artificial flowers, and for drawing 

 upon, which is really the pith of a Chinese plant, is an 

 excellent illustration of it. If the force of extension, or 

 compression, or nutrition, be greater in one direction than 

 another, a variety of forms is produced, of which the following 

 are the most worth noticing : 



1. The oblong ; in the stem of Orchis latifolia, and in the 



inside of many leaves. (Plate I. fig. 9.) 



2. The lobed (Plate I. fig. E./) ; in the inside of the leaf of 



Nuphar luteum, Lilium candidum, Vicia Faba, &c. : in 

 this form of cellular tissue the vesicles are sometimes 



VOL. I. E 



