File:Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic (1910) (14761967716).jpg

Identifier: plantanatomyfrom00stev (find matches)
Title: Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic
Year: 1910 (1910s)
Authors: Stevens, William Chase, 1861-
Subjects: Botany -- Anatomy
Publisher: Philadelphia, P. Blakiston
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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their obtaining waterand solutes two types ofaerophytes will be discussed,namely, the type where trueroots are sent forth into theair, and the type which, pro-ducing no roots, has its stemsand leaves equipped for ab-sorption. In the first in-stance, some tropical orchids,and other aerophytes lodgingon the branches of trees, sendforth roots into the air thathave an external covering ofdead tissue known as thevelamen (Fig. 45). The cellsof this tissue have many veryminute openings through theirexterior and interior wallsthrough which water passeswhen the root is wet with rainor dew. The velamen isformed by tangential division of the protoderm, beginning a short distance from the apex andgiving rise to layers of cells varying from one to eighteen ormore according to the species. The cell-walls of the velamensometimes remain thin, but usually they are thickened, eitheruniformly, or in the form of a network or spiral bands. Afterthe cells have reached maturity the protoplasts soon die, and
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J FIG. 45.—Portion of a cross section throughan aerial root of Stanhopea oculata. h, Thevelamen; i, exodermis; /, cortex; k, endo-dermis. (After Haberlandt.) ABSORPTION OF WATER BY AEROPHYTES 97

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