416 BOTANY OF THE LIVING PLANT 



seek out and even climb up the stem of other seedlings, and, coming 

 to rest, perforate the superficial cell-walls, causing a new infection. 

 It is in this way that the attack commonly appears some way up the 

 stem. This method of propagation may be continued throughout 

 the season. 



But it is found that the disease is liable to reappear in a 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 

 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. 344, b, p. 409). 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. 344, 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 

 fungus can continue its life as a saprophyte : for its activity does not 

 end with the death of the victim. There is in fact no absolute line 

 between the parasitic and the saprophytic habit. 



The risk of attack on the seedlings may be reduced by avoiding 

 too close sowing and too moist culture ; also by avoiding the use 

 of soil in which infected seedlings have been raised. Or the risk 

 may be further reduced by raising the soil before sowing to a 

 temperature that will kill all the spores that it contains. 



THE POTATO FUNGUS (Phytophthora infestans) 



The Potato Fungus is the cause of what is commonly known as 

 the " Potato Disease," which is always a risk to the crop in damp, 

 warm seasons. In the years 1845-1850 the disease already known 



