5o6 Minnesota Plant Life. 



practiced. ySome are structural, while otliers are i)liysioIogical. 

 The principal dangers to ^vhicll plants are exposed are these: 

 desiccation, inundation, fracture by wind or water, poisoning 

 by salts, over- or under-illumination, over-heating, cold, and 

 attack by other plants or by animals. The defensive adapta- 

 tions against the various inanimate harmful influences have 

 already been discussed in Chapter XL; but there remain a few 

 \\or(ls to be said concerning defense against other plants and 

 animals. The most universal method is by producing" distaste- 

 ful or poisonous substances in the tissues. Among such should 

 be counted the acids, alkaloids, tannins and resins that are 

 characteristic of many plants. By means of these substances, 

 dissolved in their juices, they make themselves objectionable 

 to grazing animals, to insects and snails, and even to parasitic 

 fungi. Sometimes the poisonous materials are projected to a 

 distance, as is true of the poison-ivy and poison-dogwood. In 

 such instances the animals are dissuaded even from approaching 

 the plant. 



Another common method of defense, especially against 

 animals, is the wearing of armor. This ordinarily takes the 

 form of i^rickles, spines, or thorns, though it may rarely appear 

 as hard scales or plates. Armor is not limited to the plants 

 of any particular region or of any adaptational group. The 

 aquatic water-fern, Sahiiiia, has its sharp-pointed hairs to deter 

 small insects from feeding upon its fruiting areas ; the meso- 

 phytic hawthorns, roses, and brambles are bristling with prickles 

 and thorns, while the xerophytic cacti and spin-ges are unap- 

 proachable in the effectiveness of their defense. Armor is par- 

 ticularly useful to desert plants, for they are most exposed to 

 attack b\' hungry animals. Less ordinarily in mind, when 

 plant-armor is mentioned, is the skin on leaves and green twigs. 

 This, while not defensive against animals of large size, is suffi- 

 cient, perhaps, to puzzle some of the smaller insects, and often 

 suffices to prevent the infection-tube of a fungus from obtaining 

 an easy entrance to the soft inner tissues of the leaf or stem. 



}klimicry is a peculiar method of defense adopted by some 

 plants. Thus, an innocuous ])lanl b\- its resemblance to a 

 l)oisonous variety often escapes injur\. In deserts some kinds 

 of ])lants are gray in color, irregular and massi\e in shape, and, 



