Nature and development of plants . ecayin bad cases of infection. When the mycelium has become wellestablished in the leaf, numerous branching hyphae extend outthrough the stomata and form at their tips little sacs or sporangia(Fig. 132, A). The formation of the sporangia is effected in afew hours, when they drop off and are carried by the wind toother plants, where they germinate at once, forming a tube thatpenetrates the leaf and rapidly spreads the disease. If the spor-angia chance to fall upon leaves that are wet by dew or rain, the 2oo REPRODUCTION OF DOWNY MILDEWS contents breaks up into
Nature and development of plants . ecayin bad cases of infection. When the mycelium has become wellestablished in the leaf, numerous branching hyphae extend outthrough the stomata and form at their tips little sacs or sporangia(Fig. 132, A). The formation of the sporangia is effected in afew hours, when they drop off and are carried by the wind toother plants, where they germinate at once, forming a tube thatpenetrates the leaf and rapidly spreads the disease. If the spor-angia chance to fall upon leaves that are wet by dew or rain, the 2oo REPRODUCTION OF DOWNY MILDEWS contents breaks up into several zoospores (Fig. 132, B-E) whichfinally come to rest and develop the characteristic tubular hvphaeof the fungi. This behavior of the sporangium is doubtless asurvival of the zoospore stage seen in the algae. It is equallvsuggestive that definite changes in the environment, as the drvair, causes the sporangium to germinate as a non-motile spore,whereas the presence of water causes the sporangium to produceseveral 3K a B :)\Fig. 132. Fig. 133. Fig. 132. Asexual reproduction of the mildew: .1, hvphae of Plasmo-para emerging from a stoma and bearing numerous sporangia. B, en-larged view of sporangium of Peronospora germinating on a dry leaf. Inthis case the sporangium behaves as a spore sending out a hypha that willpenetrate the tissues of the leaf. C, sporangium of Phytophthora germina-ting in the water and forming zoospores. D, zoospore enlarged. E,zoospore has come to rest and is forming a tube that will penetrate thetissues of the leaf as in the case of B. Fig. 133. Reproduction of the white rust, Albugo: .1, asexual stage,showing several erect hyphae forming spores. B, sexual stage which ischaracteristic of the Peronosporales in general—o, gametangium contain-ing a single female gamete which is being penetrated by tube from malegametangium, an.—After Wager. DEVELOPMENT OF PLANTS 201 Very similar in character is the grape blight, Plasmopara viti-cola. Th
Size: 1453px × 1719px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublisher, booksubjectplants
