AND THE LOWER ANIMALS. 31 



membrane, and containing in its interior a slightly 

 viscid fluid, and an exceedingly minute corpuscle, 

 called a nucleus, which again contains within itself 

 another, which is called nucleolus. These cells — 

 spherical when isolated, or polygonal because of the 

 pressure, when packed closely together — constitute 

 cellular tissue, of which we have familiar examples in 

 the pith and cork of vegetables. When these are 

 elongated in the form of spindles, and filled with 

 ligneous matter, they constitute the fibres of bark 

 and wood ; and still more developed, excavated, 

 and fused end to end, they form the vessels by which 

 the circulating fluid is transmitted. Now as these 

 three tissues — cellular, woody, and vascular — form by 

 their union, the roots, stem, branches, leaves, and 

 flowers, — in fact every organ of a plant, it follows as a 

 natural consequence, that every vegetable commences 

 its existence as a cell; moreover, this organogenical 

 law is fully borne out by comparative anatomy, some 

 of the lower forms of plants being entirely composed 

 of a single cell. 



The foregoing remarks show us why botanists 

 deem it of importance to be acquainted with the 

 phenomena of cell multiplication and development. 

 But even upon this question there has not been much 

 harmony among them. Schleiden, who is one of the 

 most distinguished botanists in Germany, put forward 

 a theory on the subject of cell-formation, which has 

 been adopted by his fellow-countrymen, and has 

 found many disciples among the botanists of other 

 nations also. According to him, cellular tissue is at 

 first quite liquid, and gradually assumes a jelly-like 

 appearance, without, however, the faintest trace of 



