484) HETEROMOEPHISM AND ALTERNATION OF GENERATIONS. 



do not forthwith escape, but swim about for a while within the wall of the 

 cell in which they originate. Then they join together into a tiny net which 

 escapes, ultimately growing to its full size (c/. vol. i. p. 36). In sexual repro^ 

 auction a much larger number (30,000-100,000) of small motile bodies (gametes) 

 escape and conjugate in pairs. Klebs has found here also that either of these 

 methods can be produced at will by altering the conditions under which the plant 



grows. 



Among the Fungi the Saprolegniacese show well-marked sexual and asexual 

 methods of propagation. These are mould-like forms which attack fish and other 

 aquatic animals upon which they are parasitic. Purely asexual reproduction occurs 

 by means of zoospores which are liberated from long, club-shaped sporangia (fig. 

 352 1); whilst sexual reproduction (which may occur upon the same plant) takes 

 place by spherical oogonia arising upon certain branches and antheridia as small 

 lateral twigs below them (fig. 352 2). The latter perforate the oogonium-wall with 

 their "fertiHzing tubes" (c/. the allied Pythium, p. 56), but curiously enough there 

 is no real fertilization. Nothing has been observed to pass from the fertilizing- 

 tubes to the egg-cells, and we must regard the process of fertilization here as 

 obsolete. The egg-cells, though unfertilized, put on cell-walls (fig. 352 3) and ger- 

 minate, as one might say, parthenogenetieally. 



In the Moulds of the family Mucorini the mycelium establishes itself upon an 

 •organic substratum and produces, at one time, long-stalked sporangia (figs. 353^ 

 and 353 ^), and at another short sac-like outgrowths which arise in pairs near one 

 another and conjugate, forming a zygospore (fig. 353 ^). Whether this or that 

 method of reproduction prevails in these Fungi depends, most probably, on external 

 conditions; indeed examples from amongst the Fungi could be adduced in which 

 careful experiment has determined that this is the case. 



In .conclusion we may inquire how is it that alternation of generations is so 

 widely distributed amongst plants, whilst in the animal kingdom it is of relatively 

 rare occurrence. We obtain an answer to this question when we consider what 

 are the distinguishing characters of those animals in which alternation of 

 generations takes place. The corals, polyps, and other animals exhibiting alterna- 

 tion are in great part sessile organisms, attached to their substratum. But when a 

 fixed organism propagates itself and distributes its kind, it must commit portions 

 of itself to the winds or to currents of water, if new regions are to be occupied; a 

 condition applying equally to plants and animals. Or, as an alternative, sexually- 

 produced progeny may be liberated from the mother-organism and take up new 

 positions. But sexual reproduction amongst fixed organisms requires rather special 

 arrangements, and even with their aid is not invariably certain. Interference with 

 fertilization may connote the extinction of the species; consequently a propagation 

 by asexual means is of great importance for such organisms. By a definite alter- 

 nation of the two methods, by a singlo act of fertilizing leading to an organism 

 capable of multiplying itself almost indefinitely by asexual spores, a numerous 

 progeny is ensured even from a single sexual union. Take the case of the Fern- 



