. Botany of the living plant. Botany. PHYCOMYCETES.—(a) OOMYCETES 419 like those of Pythium, they are motile for a time by means of two cilia (Fig. 355). Coming then to rest, the cilia are dropped : each zoospore rounds itself off, and, investing itself with a wall, puts out a hyphal tube. If this takes place on the surface of a potato leaf, as it well might do under conditions of rain or heavy dew, all is ready for the infection. This may either be by entry through the pore of a stoma or by direct perforation through the epidermal wall (Fig. 356). By either route the parasite may reach the in
. Botany of the living plant. Botany. PHYCOMYCETES.—(a) OOMYCETES 419 like those of Pythium, they are motile for a time by means of two cilia (Fig. 355). Coming then to rest, the cilia are dropped : each zoospore rounds itself off, and, investing itself with a wall, puts out a hyphal tube. If this takes place on the surface of a potato leaf, as it well might do under conditions of rain or heavy dew, all is ready for the infection. This may either be by entry through the pore of a stoma or by direct perforation through the epidermal wall (Fig. 356). By either route the parasite may reach the intercellular spaces and establish a new infection. It is not only the leaves but also the stems and tubers of the Potato- plant that may be traversed. The mycelium spreads through the tissues down the haulms tO' the tubers. A perforation of the j^oung tubers by a new infection from conidia is even possible, so long as their skin is thin. In bad cases tubers thus infected may decay at once. More commonly the hyphae enter a dormant state, and in this condition the tubers are harvested. But such tubers often decay during the winter. If, however, they are not heavily infected, and thus escape decay, and they are used as " seed " potatoes for a new crop, the young plants start infected from the first, and a recurrence of the disease is inevitable. The measures to be taken are to destroy by fire all infected haulms and leaves, to avoid carefully the use of tainted "seed" tubers ; and, as a preventive, to spray the young growing crop with suitable disinfectants, especially if the season is wet in the middle summer. But a more hopeful line of prevention is by the use of " immune varieties," which are able to resist the attack of the parasite. No mention has been made of sexual reproduction in the Potato Fungus. As a matter of fact sexual organs have not been proved to in Phytophthora infestans under normal conditions of life. Like. Fig. 354. Piece
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
