56 FERTILIZATION AND FRUIT- FORMATION IN CRYPTOGAMS. 



formation of a yoke between opposite cells which put forth lateral outgrowths 

 towards one another for the purpose; this is the reason why this kind of 

 fertilization is called conjugation, and the plants concerned are named Conjugatce. 



Similar to conjugation, but differing from it in several essential particulars, 

 is the mode of fertilization by means of a protruding outgrowth from the 

 antheridium, which pierces through the wall of the oogonium. This method is 

 observed to occur in particular in the destructive parasites comprised under the 

 name of Peronosporeae. The species named Peronospora viticola, which is repre- 

 sented in fig. 205, has attained a melancholy notoriety as a parasite on the Vine, 

 and to the same group belong Peronospora infestans, which causes the Potato- 

 disease, Cystopus candidus, known as a deadly parasite on Cruciferous plants, 

 the various species of Pythium, &c. Tubular hyphse develop directly from the 

 spores of these Peronosporese, which attack the fresh foliage, green shoots, or 

 young fruits of the particular flowering plants that they select to serve as hosts. 

 The hyphse bore into the green tissue, piercing through the cell-walls and growing 

 in the intercellular spaces, where they ramify extensively. Segmentation of the 

 hyphse by the introduction of partition-walls is comparatively rare, but very 

 frequently little suckers, called "haustoria", are sunk into the interior of the 

 living cells of the host (see vol. i. p. 165, fig. 32 ^). These hyphae infesting the 

 green tissues of the host-plant swell up at their blind extremities into globular 

 heads, and a septum is introduced in each case to partition ofi" the terminal sphere 

 from the rest of the tube, which preserves its cylindrical form. The splierical 

 cell is an oogonium, and the protoplasm forming its contents is the ooplasm. 

 The latter differentiates itself into two portions, namely, a central darker ball 

 and a clearer transparent enveloping mass. The antheridia containing the sper- 

 matoplasm develop in the form of lateral clavate outgrowths from another tube, 

 or more rarely from the same tube. These protuberances grow towards the 

 oogonium and apply themselves to its surface. As soon as the antheridium 

 touches the oogonium it sends out from the point of contact a conical or 

 cylindrical hollow process which pierces the wall of the oogonium and penetrates 

 to the dark ball in the middle of the ooplasm (see fig. 205 ^). Meanwhile the 

 protoplasm in the antheridium has differentiated itself into a parietal lining on 

 the one hand and the true spermatoplasm on the other. The antheridial process, 

 which has received the name of " fertilizing-tube ", opens at the extremity buried 

 in the interior of the oogonium; within an hour or two the spermatoplasm has 

 flowed through this channel to the ooplasm and become so completely merged 

 with it that it is no longer possible to recognize any boundary between the two. 

 A short time afterwards the fertilized ooplasm incloses itself in a thick cell- 

 membrane composed of several layers. The outermost laj'^er is usually rough and 

 warty, and in some species is even beset with spikes. The fruit thus formed 

 is unicellular and remains so. It frees itself from the now decaying oogonium — 

 thus effecting its separation from the mother-plant — and then enters upon a long 

 period of rest. The new generation developed from the fruit begins as a tube 



