Image from page 166 of "The origin of floral structures through insect and other agencies" (1888)
Identifier: originoffloralst00hensuoft
Title: The origin of floral structures through insect and other agencies
Authors: Henslow, George, 1835-1925
Subjects: Fertilization of plants Flowers -- Morphology
Publisher: New York D. Appleton
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: MSN
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cidences, that hairy herbacious plants are com-moner in dry situations and smooth ones in watery; just asroot-hairs occur in a loose sandy soil and their absence isnoticeable in a heavy one; but we do not know how thesedifferent media actually bring about these changes, thoughwe may feel assured tliat it is solely due to the environment. If we, thus, look elsewhere than in flowers for anyanalogous processes they are by no meang wanting. Forexample, it is simply the mechanical irritation brought aboutby contact with a foreignbody, probably aided bymoisture and a lessened de-gree of light, that causesthe epidermal cells of theaerial roots of the Ivy andOrchids (Fig. 42) to elon-gate into adhesive or clasp-ing hairs, so as to grasp the -pi^ 42.—Adhesive epidermal cells of roots of body for support. This is jancztwsid) ^^^ ™°*° <^only a form of the ordinary Ioot-hairs which are immediately developed when the tip is incontact with a moist soil, and each hair grips and glues itself
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138 THE STRUCTURE OF FLOWERS. to the particles of soil.* Cbatin noticed the production ofhairs when the roots came in contact with any obstacle; fbut Dr. M. T. Masters observes that the obstacle alone intheir case is insufficient without moisture, for he found thatthe roots of Mustard-seed could penetrate a stiff clay, but didnot develop any root-hairs until they came in contact withthe sides of the pot—Wherever there was a thin film ofwater investing a stone or the sides of a porous flower-potor a plate of glass, there the root-hairs abounded. Besides a nutrient or moist medium, actual growth inwater may enormously increase the length and quantity ofroot-hairs; as may be seen in the dependent roots of floatingplants of Hydrocharis, etc. ; or in the hypertrophied con-ditions of the roots of grasses when growing in water. That epidermal trichomes may be due to the irritation ofinsects is clearly seen by their appearance within the cavitiesof certain galls. J In the case, for example,
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