Metaphyta Spirogyra. . 81 



size until they touch. Fusion of the walls at the point of 

 contact takes place, and a direct communication is estab- 

 lished therefore between the two cells. Meantime the 

 protoplasmic contents of both cells contract and withdraw 

 from the cell-wall, forming a dense rounded mass in the 

 interior of either cell. This contraction is accompanied and 

 rendered possible by the expulsion of a quantity of water 

 from the cell. The contraction may take place in both 

 cells at the same time, or more usually in one cell before 

 the other. This would seem to point to some molecular 

 difference between the cells, probably of nutritive condition 

 (p. 90). The next step consists in the passage of one or 

 other of the protoplasmic masses into the other cell in which 

 latter no doubt preparatory metabolic changes are taking 

 place (fig. 21). This fact would again point to some mole- 

 cular difference between the cells. Union of the proto- 

 plasmic masses follows, and results in the formation of an 

 ovoid mass which is smaller in bulk than the sum of the 

 two masses before union, doubtless owing again to the 

 expulsion of water such a body is called in most text-books 

 a ' zygospore,' or 'resting spore.' The term 'spore,' as 

 applied to such a body, is most undoubtedly inappropriate, 

 if not positively wrong, because that name is applied to 

 cells which multiply the plant asexually (p. 95), and which 

 do not result from conjugation of two reproductive cells of 

 different sexes. l Although the conjugating cells of Spiro- 

 gyra are apparently alike, yet there can be no doubt that 

 this is a case of primitive sexual union, and therefore if we 

 decide to call the egg or female reproductive cell of a plant 

 before fertilisation an ovum, this 'zygospore' is simply a 

 fertilised ovum or embryo. It is true we cannot say before- 

 hand which of the conjugating cells is the ovum, and which 



1 The term ' spore ' is used by Vines in his Physiology of Plants to 

 designate both the asexually produced spore and the embryo, or product 

 of union of the ovum and sperm. 



*G 



