142 A TEXT-BOOk OF BOTANY 
ing upon a suitable substratum germinate and produce new 
mycelia. These spores, although asexual, are evidently not 
swimming spores, as there is no water medium for them to 
use. This method of transfer being impossible, the spores 
are scattered by currents of air, and must be correspond- 
ingly light and powdery. It is interesting to note that cer- 
tain molds that grow in the water develop swimming spores. 
While the ordinary method of reproduction through the 
growing season is by means of these rapidly germinating 
spores, in certain conditions sexual reproduction also occurs. 
Branches put forth from two contiguous mycelial threads, 
the tips of the branches being in contact (Fig. 130, A). 
Partition walls separate the tips from the main body of the 
plant (Fig. 130, B), the walls in contact become perforated, 
the contents of the two tips fuse, and a heavy-walled 
odspore is the result (Fig. 130, C).* This sexual process 
suggests that of Spirogyra (§ 69). 
80. Peronospora.—These are the downy mildews, very 
common parasites on the leaves of seed-plants. The 
mycelium is entire- 
ly internal, branch- 
ing among the tis- 
sues of the leaf, and 
piercing the living 
cells with sucker-like 
branches that rap- 
Fic. 131.—Branch of mycelium of Peronospora in idly absorb — their 
contact with two cells of a host plant, and send- contents (Fig. 131). 
ee them absorbing branches.—After Dr The presence of the 
parasite is made 
known by discolored and finally deadened spots on the 
leaves, where the tissues have been killed. 
* It is not easy to induce ./ucor to perform the sexual process, and 
in fact such a process may not often occur in nature. 
