148 



THE PLANT CELL. 



to certain cells of these filaments, a change, which results first of 

 all in the pushing out of small protrusions from the cell-walls of 

 adjacent cells (see Fig. 108). These protrusions grow out 

 laterally from the main cells until they meet, and then the 

 partitions between them become dissolved, thus leading to the 

 formation of a tubular passage joining the two cells ; the cyto- 

 plasm, chlorophyll band and nucleus of one cell then passes along 

 this passage into the other cell and fuses (with the exception of 

 the chloroplasts) with the cytoplasm and other structures of the 

 latter. The resulting mass is known as a zygote (each of the 

 original masses being the gamates), and soon takes on a thick wall 

 of cellulose ; the chlorophyll band of the receiving cell persists, 

 whilst that of the other aborts. It is usual to look upon the 



=^ 



Fig. 108. — Con JUGATiON in Spirogyra. — 1. The protoplasm in two adja- 

 cent cells has contracted, and a protrusion from each cell has already 

 been formed. 2. The protrusions have met, the intermediate wall 

 has been dissolved, and the protoplasmic contents of one cell are 

 passing into the cavity of the other. 3. Fusion (conjugation) of the 

 two masses has occurred, and the resulting "zygote" has assumed a 

 wall of cellulose. 



" fertilising " gamete as the male cell, and the other or receiving 

 gamete as the female element. At a later date the enoapsuled 

 mass is set free from the cavity of the original cell, by bursting 

 of the wall of the latter, and soon germinates ; germination 

 results in the formation of an elongated cylindrical cell which 

 divides into two and so on, so as to produce ultimately a filamen- 



