. The natural history of plants, their forms, growth, reproduction, and distribution;. Botany. THALLOPHYTA. 669 Peronosporece.—Are mostly parasitic upon Flowering Plants, and the cause of many destructive diseases. They estabhsh themselves by means of a branching, tubular, non-septate mycelium which penetrates the intercellular system of the host- plant (c/. p. 56). They propagate asexually by means of unicellular sporangia borne on branched hyphse which project from the stomates, &c., of the host (c/. fig. 38V); these sporangia (or spores as they are sometimes termed) liberate on a moist
. The natural history of plants, their forms, growth, reproduction, and distribution;. Botany. THALLOPHYTA. 669 Peronosporece.—Are mostly parasitic upon Flowering Plants, and the cause of many destructive diseases. They estabhsh themselves by means of a branching, tubular, non-septate mycelium which penetrates the intercellular system of the host- plant (c/. p. 56). They propagate asexually by means of unicellular sporangia borne on branched hyphse which project from the stomates, &c., of the host (c/. fig. 38V); these sporangia (or spores as they are sometimes termed) liberate on a moist sub- stratum a number of swarm-spores (figs. 381 ^-I^'*') which originate new Fig. 380.—Swarm-spores in Saprolegniacese and Cbytridiacese. 1 AcMya prolifera. 2, s, 4 Successive stages of swarm-spore-formation in Achlya prolifera. ^ Chytridium OIUl, parasitic on the oogonium of the Alga (Edogonium; development of swarm-spores, 6 Saprolegnia laetea. ? Development of swarm-spores in the same. (Partly after De Bary and Pringsheim.) ix20; 2, =,4x400; 6x300; «xlOO; i'x300. Sexual reproduction also takes place by the formation of oogonia and tube-like antheridia. The latter become attached to the former (fig. 381^), and, putting out fertilizing tubes which penetrate to the egg-cell within the oogonium, transmit their spermatoplasm. No spermatozoids are differentiated, but the spermatoplasm travels en masse. The fertilized egg-ceU enters on a resting stage, and when it germinates may either give rise to swarm-spores ( Cystopus) or grow at once into a new plant (Pythium, Peronospora). To Phytophthora infestans is due the well- known Potato-disease. The Fungus attacks the foliage and reproduces abun- dantly asexually. Later, its mycelium penetrates to the tubers and passes into a dormant state there. Consequently when stored these potatoes go bad, and if. Please note that these images are extracted from scanned page images that may have been digitally enhanced for reada
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectbotany, bookyear1895
