10 ORGANOGRAPHY. BOOK I. 



have developed at the rate of near 4,000,000,000 per hour, 

 or of more than sixty-six milhons in a minute. 



The bladders of cellular tissue are always very small, but are 

 exceedingly variable in size. The largest are generally found 

 in the gourd tribe (Cucurbitaceae), or in pith, or in aquatic 

 plants ; and of these some are as much as the -^l of an inch in 

 diameter ; the ordinary size is about the ^^o or the 3^1^, and 

 they are sometimes not more than the toVtj' Kieser has 

 computed that in the garden pink more than 5100 are con- 

 tained in half a cubic line. 



Cellular tissue is found in three essentially different states, 

 the membranous, the Jibrousy and the vasiform. 



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 considered 

 the basis of vegetable structure, and the only form indispens- 

 able to a plant. Many plants consist of nothing else ; and 

 in no case is it ever absent. It constitutes the whole of Mosses, 

 Algae, Fungi, Lichens, and the like; it forms all the pulpy 

 parts, the parenchyma 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 membranes 01 

 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 issue ; 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 con- 

 sidered to be caused by the compression or extension of such 

 spheroids. 



When a mass of spheroidal bladders is pressed together 

 equally in all directions, rhomboidal dodecahedi'ons are pro- 

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

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



