FERTILIZATION AND FEUIT-FOEMATION IN CRYPTOGAMS. 59 



spermatoplasm with the ooplasm occurs. The protoplasts set aside for the 

 purpose of coalescence forsake the cell-interiors when they have attained to 

 maturity, or at least one of the sexual cells liberates its protoplasm so that it 

 reaches the other unfettered and is enabled to effect a union of their two masses. 

 For this result it is necessary for a part of the cell-membrane enveloping the 

 protoplasm in question to be previously removed, for otherwise it would not 

 be possible to effect the kind of union to which the phrase coalescence of 

 protoplasm is properly applicable. On the other hand, many cases exist in which 

 there is no obvious perforation of the wall, although the changes usually fol- 

 lowing true fertilization take place. Under these circumstances it is difficult to 

 resist the view that if fertilization (i.e. a fusion of protoplasts) really happens 

 (as to which difference of opinion still prevails) it is accomplished by means 

 of osmosis. With this qualification we may say that fertilization by means of 

 osmosis is observed in its simplest form in the Erysiphese, popularly known as 

 Mildews, in the Moulds allied to Aspergillus and Penicillium, a description of 

 which in relation to their methods of spore-formation is given on pp. 21, 22, and 

 in several Discomycetes, including the curious Fungus named Ascoholus, which wiU 

 be dealt with more thoroughly when we come to the subject of the mechanisms 

 for dispersing spores. 



The Mildew occurring on the surfaces of green foliage-leaves appears under 

 the microscope as a peculiar kiad of mycelium. The hyphse, which are filiform, 

 colourless, and densely interwoven, do not penetrate into the intercellular spaces 

 of the tissue of the host-plant, but satisfy themselves with sinking Httle suckers 

 into the superficial cells of the leaves and stem (see vol. i. p. 165, fig. 32 ^). Here 

 and there these hyphal tubes rise erect from the substratum and abstrict monili- 

 form rows of spores; others put forth short, lateral outgrowths which become 

 partitioned off by the insertion of a transverse wall in each, so that the protoplasm 

 in the outgrowth is shut off from the rest of the protoplasm in the tube. Some 

 of these latter structures are oval or club-shaped, and they contain ooplasm and 

 are to be considered as oogonia; the others are cylindrical and sometimes bent 

 into the form of hooks, and they contain the spermatoplasm and constitute 

 antheridia. In a few species the upper, somewhat swollen end of the outgrowth 

 filled with spermatoplasm — i.e. the antheridium — bends over the top of the 

 oogonium and attaches itself closely thereto, without, however, sending any 

 special fertilization-tube into the interior of the oogonium; in other Fungi of 

 the Mildew family both cells — the oogonium as well as the antheridium — are 

 spiral and are coiled round one another, and at the same time pressed tightly 

 together. On the assumption that a true fertilization now occurs, this must, as 

 already indicated, be by a diffusion of the spermatoplasm through the cell-mem- 

 branes to the ooplasm, causiag a change in its ultimate structure which corresponds 

 to fertilization. The ooplasm is thereupon converted into an embryo. The cell 

 inclosing the embryo neither dissolves nor severs itself from the parent-hypha, but 

 divides and becomes differentiated into an upper swollen cell and a lower short,. 



