Comparative Physiology. 197 



flowering plants ; but, lying in no regular direction in regard to 

 the other vessels and fibres, have been commonly overlooked. 

 They resemble the capillary vessels of the lymphatic system of 

 animals ; but, in elementary structure, do not differ from vege- 

 table cellular tissue ; the branching character being due to an 

 arrangement of the primary cells, in which these tubes ori- 

 ginate." 



The tissues he thinks, to a certain extent, transformable ; all 

 the tissues have been said to be formed of the cellular. He 

 is of opinion, that in animals it is not so much from the cellular 

 matter, ordinarily so called ; the cellular, like all the other tissues, 

 being preceded by the existence of a semitransparent gelatinous 

 matter, of which the entire embryo seems at an early period to 

 be formed, and which may be regarded as a vesicular sti-ucture re- 

 sembling that of the inferior plants and animals. The cells of 

 which it consists give origin simultaneously to the various kinds 

 of tissue, and disappear as they are evolved. Cellular tissue may 

 be transformed into the other vessels, but will retain the form 

 impressed on it. A similar gelatinous semiorganised matter is 

 found in the peculiar juices (cambium) of plants, which is ex- 

 ceedingly apt to assume the appearance of membrane, and is 

 likely similar to that of animals. The opinion seems very 

 general that elongated cellules, dotted ducts, &c., which follow 

 the woody fibre up the stem, and extend into the centre 

 horizontally by the medullary rays, are formed of cells united, 

 compressed, and perforated ; and that the spiral cells, or fibro- 

 membranous tissue, as also the different modifications of spiral 

 vessels, are formed from cells round which spiral threads are 

 coiled in various ways, and to various extent ; simple, and in- 

 terrupted, or dotted, as in some varieties, up to the perfect spiral 

 vessel, wliich is coiled all round. Some have ascribed the same 

 origin to woody fibre, but its toughness and elasticity have been 

 thought distinct from modifications of common cells. They are 

 found formed in bundles, probably from what might have been 

 round cells, but elongated at the time of formation from some 

 j)eculiar action on the granules. Mirbel says, in watching the 

 evolution of Marchantz'a, he found the cells elongate, and spiral 

 fibres to be formed on them; these spiral cells, and the ducts of 

 ferns, he thinks a transition forwards to the true spiral vessel. 



" Membrane and fibre compose the vegetable tissues ; mus- 

 cular fibre and nervous matter are peculiar to animals, and, for 

 the most part, restricted to those parts of the fabric subservient 

 to the functions purely animal, namely, sensation and voluntary 

 motion. One of the characteristics of animals is the possession 

 of a digestive cavity, to store up the food for the continued 

 supply of the absorbent system, and cause it to undergo a 

 certain degree of preparation ; this addition to the absorbent 



