. Foundations of botany. spines are dead and drystipules. exposes a sharp, jaggedtube full of irritating fluid. These tubular hairs, with their poisonous contents, will be found sticking in the skin of the hand or the faceafter incautious contact with nettles, and the violent itchingwhich follows is only too familiar to most people. 419. Cutting Leaves. — Some g-rasses and sedges aregenerally avoided by cattle because of the sharp-cuttingedges of their leaves, which will readily slit the skin ofones hand if they are drawn rapidly through the the microscope the margins of
. Foundations of botany. spines are dead and drystipules. exposes a sharp, jaggedtube full of irritating fluid. These tubular hairs, with their poisonous contents, will be found sticking in the skin of the hand or the faceafter incautious contact with nettles, and the violent itchingwhich follows is only too familiar to most people. 419. Cutting Leaves. — Some g-rasses and sedges aregenerally avoided by cattle because of the sharp-cuttingedges of their leaves, which will readily slit the skin ofones hand if they are drawn rapidly through the the microscope the margins of such leaves are seento be regularly and thickly set with sharp teeth like thoseof a saw (Fig. 247, c, d). HOW PLANTS PROTECT THEMSELVES 351 420. Weapons of Desert Plants. — In temperate regions,where vegetation is usually abundant, such moderatemeans of protection as have just been described are gener-ally sufficient to insure the safety of the plants which havedeveloped them. But in desert or semi-desert regions the. Fig. 247. — Stinging Hairs and Cutting Leaves. (All much magnified.) a, stinging hairs on leaf of nettle ; b, bristle of the bugloss ; c, barbed margin of a leaf of sedge ; d, barbed margin of a leaf of grass. extreme scarcity of plant life exposes the few plants thatoccur there to the attacks of all the herbivorous animalsthat may encounter them. Accordingly, great numbers ofdesert plants are characterized by nauseating or poisonousqualities or by the presence of astonishingly developedthorns, while some combine both of these means of defense. 352 FOUNDATIONS OF BOTANY 421. Offensive or Poisonous Plants. — A disgusting smellis one of the common safeguards which keep plants frombeing eaten. The dog fennel (Fig. 227), the hounds-tongue[Cynoglossum)^ the Martynia, and the tomato-plant arecommon examples of rank-smelling plants which are offen-sive to most grazing animals and so are let alone by , as in the case of the jimson weed [Datura),the
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