28 THE ELEMENTARY STRUCTURE OP PLANTS. 



globular mass (Fig. 89), which at length is coated with cell-mem- 

 brane, and so becomes a cell of considerable sizC; Li Zygnema 

 (Fig. 635) the whole contents of two cells are united, and give 

 rise in a similar way to one new cell. 



30. In the higher or flower-bearing division of plants, this process 

 of original or free cell-formation occurs only in the sac in which the 

 embryo is formed. The first cell of the embryo originates in this- 

 way ; but all the subsequent growth is effected by a different pro- 

 cess. In the simplest grade of plants it occurs moi'e frequently, 

 but only in the formation of those bodies which in them take the 

 place and fulfil the ofhce of seeds ; that is, which serve for repro- 

 duction. 



31. It appears, therefore, that the azotized or nitrogenous mate- 

 rial, the proteine, plays the most important part in the formation 

 of cells. The layer of protoplasm, with its dehcate coating, the 

 primordial utricle, precedes the proper ceU-membrane, and in 

 some unexplained way causes the latter to be deposited on its sur- 

 face. And these soft nitrogenous parts are the seat of the whole 

 vital activity of the cell. The wall of cellulose may be regarded 

 as a kind of protecting coat or shell, which constitutes the per- 

 manent fabric of the plant, but is aUve only so long as the living 

 protoplasmic lining remains. 



32. In a growing young cell, the walls enlarge much faster than 

 the nucleus, and the latter soon ceases to grow at all. It is there- 

 fore left in the centre, or else remains adherent to the wall on one 

 side, where traces of it may often for a long time be detected ; or 

 more commonly it dissolves and disappears altogether. At length, 

 in older cells, the hquid contents and the protoplasmic lining also 

 disappear, and only the walls of cellulose remain as the permanent 

 vegetable fabric. The fabric of plants, however, as has already 

 been stated, is not built up by original cell-formation, but by 



33. CeII-3Iultiplication. A living cell, formed in Avhatever manner, 

 has the power of multiplying itself by dividing into two, these again 

 into two more, and so on. By this process the single cell, which 

 each vegetable begins with, gives rise to the embryo or rudimen- 

 tary plantlet contained in a seed ; and by it the embryo in germina- 

 tion develops into a seedling, and the seedling into the herb, shrub, 

 or tree. Vegetable growth accordingly consists, — 1st, of the growth 

 or expansion of each cell up to its full size, which ordinarily is very 

 soon attained ; and 2d, of what is called their merismatic multiplica-' 



