. A textbook of botany for colleges and universities ... Botany. 688 ECOLOGY. suberized. Branch veins and veinlets usually are much more numerous in xerophytic leaves than in the leaves of shade plants and hydrophytes. Parasitism and vascular development. — When Orobanche fasciciUata grows parasitically on an Artemisia root (fig. 1083), the latter often is stimulated to unusual development, the hadrome in particular being subject to extensive enlargement. In the haustoria of Melampyrum, tracheids develop only after attachment to a host plant.' Leaves infested by the parasitic fungus, Peronospo
. A textbook of botany for colleges and universities ... Botany. 688 ECOLOGY. suberized. Branch veins and veinlets usually are much more numerous in xerophytic leaves than in the leaves of shade plants and hydrophytes. Parasitism and vascular development. — When Orobanche fasciciUata grows parasitically on an Artemisia root (fig. 1083), the latter often is stimulated to unusual development, the hadrome in particular being subject to extensive enlargement. In the haustoria of Melampyrum, tracheids develop only after attachment to a host plant.' Leaves infested by the parasitic fungus, Peronospora, sometimes develop entirely new bundle tracts, certain primordia that commonly grow into mesophyll developing instead into vascular tissue. In insect galls of Viiis (fig. 823) there is a vast increase in hadrome, there being about the larval chamber a. festoon of this tissue developed from the cortex. Some- times in vascular tracts infested by para- sites, parenchyma cells adjoining the enlarged vessels become hypertrophied, bulging out into the vessels as tyloses (p. 695)- Miscellaneous reactions of vascular tissue. — A potato tuber usually decays after giving rise to new tubers or rhi- zomes, which withdraw the food it had accumulated. But if a tuber is planted at the ground level in such a way that sprouts developing in the air are con- nected with the developing roots only through the old tuber (fig. 1046), the latter not only lives another season, but many of its mature cells become once more meristematic. Among such re- juvenating tissues conductive elements play the most important part, many parenchymatic cells growing into tra- cheids and becoming of importance in conducting water and salts to the growing shoots. Similarly, when a le^f of Torenia is placed upright in the soil, it gives rise to a shoot; the dorsiventral petiole develops into a radially symmetrical organ, and new vascular bundles develop from parenchyma, forming a vascular cylinder comparable to th
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
