FVNGI. SAPROLEGNIEAE. 



97 



which grows away from it into the air, and another which penetrates it ; the former 

 grows rapidly in length and thickness and puts out a number of branches near its base 

 (Fig. 59) ; the latter forms copious slender ramifications which spread through 

 the substratum like rhizoids. Rhizoids are also formed on the branches outside the 

 substratum, and penetrate into it (Fig. 59). Finally, the Saprolegnieae usually cover 

 animal or vegetable organisms, dead insects especially, which have fallen into the water, 

 but sometimes fishes also, with dense radiating tufts of filaments. Each individual plant, 

 which branches in a shrub-like manner outside the substratum and is fixed in it by 

 rhizoids, is composed at first of an unsegmented tube, in which irregularly disposed 

 transverse septa are eventually formed. Asexual propagation is effected not by mo- 

 tionless gonidia but by swarm-spores, as might be expected in plants living in the 

 water. The swarm-spores are formed on branches the contents of which have been 

 isolated by transverse walls. Sometimes several such transverse divisions arise, and 

 then every cell can produce swarm-spores. The swarm-spores are formed by simul- 

 taneous division of the contents of a cell into a great number of parts, each of which has 

 a nucleus (Fig. 60, A}. Then the cell opens at its apex, and the spores are ejected and 

 swarm in the water and disperse ; or they 

 remain at first heaped in a non-motile state 

 before the aperture of the cell, and each 

 spore clothes itself with a delicate membrane, 

 which it soon however casts off and swarms 

 away (Fig. 60, B}. In some cases the swarm- 

 spores, which are usually separated from one 

 another in the mother-cell by films of granular 

 protoplasm with a capacity of swelling, be- 

 come invested with delicate membranes while 

 they are still in the mother-cell and form a 

 kind of parenchyma there, and escape by 

 swarming through a number of holes in the 

 mother-cell. These various modes of forming 

 spores, which have been used to distinguish 

 genera and species, may occur together, as 

 Pringsheim has shown, on the same plant, as 

 in the genera Saprolegnia and Achlya. When 

 the spores have swarmed out of the terminal 

 cell of a branch in Saprolegnia^ the trans- 

 verse septum arches outward and grows into 

 a new receptacle which takes the place of the one just emptied ; in Achlya a new lateral 

 branch is formed beneath the septum and becomes a new zoosporangium. The swarm- 

 spores when they germinate give rise to plants of the same kind, whiqh on a nidus, 

 such as a dead fly, form at first only asexual organs of propagation, zoosporangia. 

 Sexual organs make their appearance only towards the close of the period of vegetation, 

 and in their regular and perfect form upon the same plant as the zoosporangia. The 

 oogonia and antheridia are of the same form as in the Peronosporeae. The former 

 usually arise as spherical swellings of the extremities of branches, but not unfrequently 

 on some point in the middle of a tube. The formation of the oosphere is not as in the 

 Peronosporeae ; the whole of the protoplasm of the oogonium is used up for this pur- 

 pose, and there is therefore no separation of periplasm. Only one oosphere is formed 

 in the oogonium of Aphanomyces, in others two or more ; "their development 

 runs through three stages, the aggregation, the separation, and the rounding off; 

 but we cannot describe these processes here in detail. The antheridia are formed at 

 the same time as the oogonia. Thinner branches arise usually on the filament that 

 ^ bears the oogonium, and grow towards the oogonium and attach themselves closely to 

 it. The terminal cell cut off at the extremity of such a branch by a transverse wall is 



w 



a, 



FIG. 59. A plant of Achlya prolifera, twenty-four 

 hours old, grown from a zoospore on the larva of a gnat 

 (after de Bary). The surface of the larva is indicated 

 by the line aa, the plant is attached to it by the primary 

 rhizoids r\ . The portion of the germ-tube outside the 

 substratum has put forth branches like a tree, and the 

 branches send down secondary rhizoids (r) into the sub- 

 stratum. 



