THE CELL DOCTRINE. 35 



not surrounded by opaque or colored bodies, — and 

 continuing thus after pressure. This substance, which 

 frequently occurs in plants, Schleiden calls vegetable 

 gelatin, and considers as slight modifications, pectin, 

 the basis of gum tragacanth, and many of the sub- 

 stances usually enumerated under the term vegetable 

 mucus. It is this gelatin vphich is ultimately, 

 through the agency of the nucleus, converted into 

 the actual cell-wall, or structures which consist of it 

 in a thickened state, and into the matter of vege- 

 table fibre. 



There are two situations in the plant in which 

 new organization may be observed most easily and 

 clearly, in consequence of there being cavities closed 

 by a simple membrane, Ist, in the large cell, which 

 subsequently contains the albumen of the seed, the 

 embryonal sac, and 2d, in the extremity of the pollen 

 tube, from which the embryo itself is developed. The 

 embryonal sac never contains starch originally, but 

 probably in most instances the saccharine solution or 

 gum. The pollen always contains starch, or repre- 

 senting it, a semi-granulous substance identical with 

 the small granules in the gum above alluded to, which 

 Schleiden calls mucus. 



In both of these situations the above-mentioned 

 minute mwcMs-granules are very soon developed in 

 the gum, upon which the solution, previously homo- 

 geneous, becomes clouded and more or less opaque. 



Single, larger, more sharply defined granules next 

 become apparent, A, Fig. 2, constituting the nucleoli, 

 and soon after the cytoblasts or nuclei, B, appear, look- 

 ing like granulous coagulations about the granules. 



