BASIDIOMYCETES 447 



The most striking modifications are seen in the Phycomycetes, 

 which bear similarity to non-septate Algae in having motile zoo- 

 spores and gametes. Among the Oomycetes the aquatic origin 

 is clearly retlected in the germination of the sporangia, where each 

 bursts and gives rise to zoospores motile in water (Figs. 303, 308, 

 pp. 404, 409). The sporangium in fact appears to be like the sporan- 

 gium seen in many Algae, but reduced in size and detachable, thus 

 permitting of its distribution by air currents, though its germina- 

 tion is still carried out in water. Similarly in the Zygomycetes the 

 large sporangium of Mucor is also effective for water-distribution of 

 its spores, though they are not themselves motile (Fig. 313, p. 4 J -0- 

 But within the family of the Mucorales the sporangium' is liable to be 

 reduced in size, with increase in its numbers, till in Chaetocladmm 

 and Piptocephaiis it matures as a single detachable cell : in fact it is 

 an air-borne conidium, the large numbers of these conidia compen- 

 sating for the reduction from the sporangial condition. The Oomycetes 

 and Zygomycetes thus suggest a parallel progression from sporangia 

 producing numerous spores to small wind-borne bodies, ranking as 

 conidia. The profuse propagation of the Eumycetes by various 

 types of detachable unicellular buds, also called conidia, acts biologi- 

 cally in the same way, as a means of subaerial propagation and dis- 

 tribution. But the phyletic origin of such conidia was probably 

 from a source distinct from that of the Phycomycetes. 



On the other hand, the sexual organs of Pythium and Cystopus 

 correspond in form and general characters to those of the Siphonales, 

 such as Vaucheria. But in place of a dehiscent antheridium, shedding 

 spermatozoids motile in water, a fertilising tube is found, which, 

 like a pollen-tube, transfers its contents directly to the ovum (Fig. 

 310, p. 410). It is in fact an antheridium, which being subaerial in its 

 development does not dehisce to set free motile gametes. In the 

 Mucorales the zygospores may very probably be regarded in the same 

 way, but referable to a more primitive state of sexuality. Two 

 distal gametangia, instead of dehiscing and their separate gametes 

 fusing, conjugate as a whole, producing the coenocytic zygospore 

 (Fig. 314, p. 416). Such examples accord with the general reference 

 of Land-living Plants in their origin to an aquatic ancestry ; and 

 they illustrate how the modifications in Thallophytes may run 

 parallel with those of organisms higher in the scale. They go far to 

 support the general thesis that Plant-Life originated in the water 

 and spread later to land-surfaces. 



B.B. 2F 



