i5° 



PART II. — VEGETABLE HISTOLOGY. 



or decay of the old cell-wall. In this way ascospores are formed 

 in Lichens and in some Fungi, Fig. 394, and pollen grains in the 

 interior of the anther, Fig. 395. 



2 Rejuvenescence of Cells. This consists of the aggregation 

 of the protoplasm of the cell into a rounded mass, its escape 

 through the cell-wall, and the subsequent formation for itself of 

 a new cell-wall. Commonly, during the period which intervenes 

 between the escape from the old cell and the formation of a new 

 cell-wall, the protoplasm is endowed with the power of locomo- 

 tion, and moves about either in an amoeboid manner, or by 

 means of cilia. This mode has been observed only among some 

 of the lower forms of plant life, Fig. 396. 



3. The Union of Cells. This consists in 

 the coming together and blending of the 

 protoplasms of two distinct cells to form a 

 new one. This mode of new cell formation 

 is confined to the reproductive process, and 

 is essentially what constitates reproduction 

 in all plants that reproduce by a truly sexual 

 process. There are two principal varieties 

 of it: 



a. That in which the uniting cells are to 

 all appearance alike. This is observed only 

 among certain low forms of vegetable life, as 

 in Mucor (a mould) and in the Diatoms, Des- 

 mids, Mesocarpus and Spirogyra, all simple 

 forms of algae, Fig. 397. This process is 

 called conjugation. Fig. 397. 



Fig. 3Q7. — Portions of two filaments of Spirogyra, showing cell formation by conju- 

 gation. The protoplasm from the upper cell of the filament to the left has passed over 

 into the corresponding one to the right. 



The same process, in an earlier stage, is shown in the two cells below these, and in 

 the lowest ones it has just begun, by the formation of two protuberances on the adjacent 

 sides of the cells. Magnified about 250 diameters. 



b. That in which the uniting cells are unlike. In this case 

 one cell (the male or sperm cell) is commonly not only smaller, 

 but more active than the other, which is called the female or 

 germ cell. This mode is illustrated in the reproduction of all 

 the higher plants, and is called fertilization. 



It will be seen that in all these cases new cells are formed 

 from pre existing ones ; that living organisms are derived from 



