248 PRACTICAL LESSONS IN SCIENCE. 



animals and are distributed in their wanderings. The seeds of 

 other plants are inclosed in edible fruits and are scattered by 

 the animals using the fruit for food. In some cases the seed 

 pod at maturity opens with a jerk throwing the seeds some 

 distance. 



Many plants, as the strawberry, multiply not by means of 

 seeds alone, but send out runners which, rooting at the tips, de- 

 velop into new plants. Again other plants send up new plants at 

 intervals along an underground stem; such plants aremore com- 

 monly found in sandy soils. 



In general plants seem to spread out as much surface as possi- 

 ble for evaporation, but in the case of the cactaceae and some of the 

 spurges living in hot dry regions, this surface is much reduced, 

 seemingly as a protection against excessive evaporation. Many 

 plants living in cold climates are protected from cold by a thick 

 growth of hairs, which frequently disappear when such plants are 

 acclimatized to warm regions. Buds, the embryos of the next 

 season's branches and flowers, are often furnished with coats of 

 down or wool, and covered with thick varnished scales so that 

 they are protected from moisture and sudden changes of tem- 

 perature. Again, thorns, spines, prickles, stinging hairs, acid 

 and bitter juices, etc., seem to serve plants as a defense against 

 animals. 



Some trees, as the red or soft maple, when cultivated will grow 

 vigorously on the highland, but when left to their own resources 

 other trees soon crowd them into the lowlands along the streams, 

 resulting in more or less change of form. This is an illustration 

 of the struggle going on between plants everywhere, by which 

 many seeds have no chance to grow, and many plants which begin 

 to grow are smothered out or stunted by stronger plants or are 

 crowded into undesirable localities. 



Some spreading plants creep out over rocks or barren places, 

 dust collects along their stems, and mingling with the decaying 

 leaves forms a soil in which other plants may grow, as illustrated 

 by vegetation advancing over brick walks, and similar spaces. 

 Often soil is formed in this way that is deep enough to support 



