54 FERTILIZATION AND FRUIT-FOEMATION IN CRYPTOGAMS 



escaped from the cells of the mother-plants into the surrounding water, and at the 

 time of fertilization they are destitute of any special coverings of their own. In 

 the plants to be dealt with next, on the other hand, the ooplasts at the time of 

 fertilization are still in connection with the mother-plant. The cell-membrane, 

 which maintains this union, persists as an envelope to the protoplasm which is to 

 undergo fertilization. There are two ways in which a fertilizing protoplast may 

 exercise its influence upon a protoplasmic body thus inclosed in a cell-membrane. 

 Either a piece of the envelope is broken through and a free passage made for the 

 spermatoplasm to the ooplasm, or else, if a true fertilization takes place, it must be 

 by osmosis through the envelope. 



The solution and removal of part of the cell-membrane enveloping the ooplast, 

 and the opening up of a passage in which the spermatoplast can unite with the 

 ooplast, is observed to occur in the Mould-fungi known as Mucorvni, and also in 

 the innumerable little green and brown water-plants which, on account of their 

 characteristic mode of fertilization, have received the name of Gonjugatce. In these 

 plants the coalescence of the two kinds of protoplasts is always preceded by a 

 process of "conjugation", that is to say, the envelopes surrounding those protoplasts 

 come in contact and grow together, and a special cavity is thereby created in which 

 the fusion of the protoplasts can take place. This method of fertilization is shown 

 in the clearest manner in fig. 204 i^^.s,*, the instance being that of Sporodinia grandis, 

 a Fungus belonging to the Mucorini. Two more or less parallel tubular hyphse put 

 forth lateral protuberances (fig. 204 ^) which stretch out towards one another until 

 their free ends come into contact and cohere. As soon as this union is eflfected, a 

 transverse wall is formed on either side of the plane of contact, and it is now 

 possible to distinguish in the limb connecting the two hyphse a median pair of cells 

 supported by the two basal portions of the outgrowths (see fig. 204^). The con- 

 necting limb is usually likened to a yoke (tvyi"). The wall arising from the junction 

 of the outgrowths, and now separating the two cells in the middle of the yoke, 

 dissolves, thus producing a single cell-cavity (instead of the two), which is called a 

 " zygogonium ". The two protoplasts inhabiting the pair of cells were hitherto 

 separated, one being derived from the hypha to the right, and the other from the 

 hypha to the left; they are two different individuals, but, upon the dissolution of 

 the wall between them, they coalesce within the zygogonium. This coalescence is 

 to be looked upon as the act of fertilization. The membrane of the median cell, 

 which surrounds the blended mass of protoplasm, thickens, and, in the selected 

 instance of Sporodinia grandis, becomes warted, whilst in Mucor Mucedo (fig. 

 193 8) it becomes rough and wrinkled, and in other Mucorini even spinose. It also 

 acquires a decided dark coloration. Lastly, the dark median cell detaches itself 

 from the basal portions of the original outgrowths, which have held it up to that 

 time, and thus becomes free and independent (see fig. 204 *). It then drops just as 

 a cherry does from the twig of a tree, and, like the cherry, it must be designated as 

 a fruit, although it consists of a single cell only. Fruits of this kind have received 

 the name of " zygotes ". 



