PHYCOM YCETES— {a) ( )( )M Y( KTES 



405 



cell-walls, causing a new infection. It is in this way that the attack 

 commonly appears some way up the stem. This method of propaga- 

 tion may be continued throughout the season. 



It is found that the disease is liable to reappear in the following 

 year in seedlings grown on soil that has been badly affected before. 

 This has been explained by the discovery of an alternate mode of 

 sexual propagation, which produces oospores : these retain their 

 vitality through the winter. If an infected plant be kept moist, 

 or even immersed in water for a few days, the hyphae begin to form 



Fig. 304. 



Development and fertilization of the ovum of Pythium. The granular protoplasm 

 of the oogonium (c) collects into a ball and the antheridium sends in a fertilising 

 tube. In (b) and (c) transmission of contents into the ovum is shown, (d) The 

 ovum has formed a cell-wall, and lies loosely in the oogonium. Highly magnified. 

 (After Marshall Ward.) 



swellings at their ends, like the sporangia but larger (oogonia). Pre- 

 sently in these the protoplasm begins to draw away from the wall, 

 and rounds off as a central sphere (the ovum). Meanwhile another 

 branch, or the end of another hypha, grows up with a smaller swelling, 

 which is also cut off by a septum (the antheridium). It comes into 

 contact with the first, and puts out a slender tube which penetrates 

 the cell-wall and extends to the ovum (Fig. 304, b). This is the 

 fertilising tube, which transmits its contents into the ovum. After 

 fertilisation the zygote surrounds itself with a cell-wall, which soon 

 thickens, but retains a smooth surface. This is the oospore, which 

 can retain its vitality through the winter (Fig. 304, d). The oospores 

 will not germinate at once, but require a period of rest. In the spring 

 under favourable conditions the thick wall bursts and a hypha is 

 produced, which soon develops sporangia and zoospores as usual. 

 The initial infection of cultures of seedlings in each year is from this 

 source, the resting spores being present in the soil. But besides this 

 the fundus can continue its life as a saprophyte . for its activity does 



