434 A TEXTBOOK OF BOTANY [Ch. X 



which produce numerous highly specialized sperm-cells. 

 In fertilization, a sperm cell reaches the egg cell through 

 a gap which opens in the spiral envelope, and the resulting 

 oospore is a resting spore. It remains surrounded by the inner 

 walls of the lime-incrusted spiral envelope of which the 

 outer parts degenerate ; and thus is formed a stony, spirally- 

 marked, nut-like shell. The two prominent genera are Char a, 

 which comprises the coarser kinds, distinguished by presence 

 of a sheath of cells around the long axial cell of the stem, 

 and Nitella, which is more slender, lacking the sheath. 

 They spread only through incidental movements of the water, 

 without special adaptations. 



Ecologially the Charales are typical sedentary hydro- 

 phytes. They really represent, like Hydrodictyon, greatly 

 overgrown micro-hydrophytes. Phylogenetically their posi- 

 tion is very uncertain, since no connecting links with other 

 groups exist. They seem to come nearest to the Siphonales, 

 of which they perhaps represent a very ancient, and now 

 highly specialized, offshoot. 



Class 6. Phycomycetes : the Algal Fungi 



This is one of the five large groups of Fungi (page 399), 

 and it belongs in this place because of its obvious relationship 

 to the Green Algae, especially the Siphonales, of which it 

 is clearly an hysterophytic offshoot. It includes the most 

 familiar of the bread molds, the Water Molds which 

 surround the bodies of floating insects and small fish, and 

 the Downy Mildews of leaves. They are partly saprophytic 

 and partly parasitic; and some are the cause of typical 

 and destructive plant diseases. 



The plant body consists of a colorless mycelium of pro- 

 fusely branching ccenocytic hyphae, which ramify and pene- 

 trate everywhere over and through the substratum or host, 

 from which food is absorbed in the characteristic manner 

 (page 83) . As in Siphonales, these hyphae show often marked 

 protoplasmic streaming. They reproduce in part by asexual 



