182 



PLANT PROTECTION. 



In the last number of this journal it 

 was shown how plants seek to avoid the 

 visits of unsuitable insects to their flow- 

 ers. This is one means of protection, but 

 there are many others which are even 

 more striking and vital. It is supposed 

 by many that plants are helpless beings, 

 which must submit to all sorts of unfa- 

 vorable conditions which come upon 

 them. This is far from true, for while 

 plants as a rule are fixed and unable to 

 escape from danger by flight, still they 

 have very many ways of helping them- 

 selves. 



Prominent among the dangers which 

 come to active green plants are those 

 which arise from too intense light, which 

 may destroy the delicate working sub- 

 stances. Since the leaves are the great 

 working organs in the manufacture of 

 food, they are especi-ally equipped for 

 protection. Those leaves which must 

 work in exposed places have many de- 

 tails of structure which are evidently for 

 guarding them against the ill effects of 

 too intense light. The most striking 

 adaptations, however, are those which 

 have to do with protective positions. Un- 

 der ordinary circumstances leaves are 

 placed so that their flat faces are exposed 

 to the most intense light. In some cases 

 this is so great a danger that the leaves 

 are set edgewise, the edges being direct- 

 ed upwards and downwards. When a 

 plant assumes this habit, the leaves are 

 said to be in a profile position, and the 

 plants are sometimes called "compass 

 plants." The latter name has come from 

 the fact that such leaves usually point 

 north or south, and once it was assumed 

 that this position was in response to 

 some mysterious magnetic influence. It 

 is found, however, that it is merely an ef- 

 fort on the part of the plant to protect its 

 leaves from the intense light of midday, 



and at the same time to expose them to 

 the morning and evening rays of much 

 less intensity. If a leaf is to be placed 

 with its edge upwards and its flat faces 

 east and west, it follows of necessity that 

 it will point either north or south. 



Some leaves, however, have the power 

 of shifting their position according to 

 their needs, directing their flat surfaces 

 toward the light, or more or less inclining 

 them according to the danger. Perhaps 

 the most completely adapted leaves of 

 this kind are those of the "sensitive 

 plants," whose leaves respond to various 

 external influences by changing their po- 

 sitions. The sensitive plants abound in 

 dry and hot regions, and one of the best 

 known is represented in our illustration. 

 It will be noticed that the leaves of this 

 Mimosa are divided into very numerous 

 small leaflets, which stretch in pairs along 

 the leaf branches. When the time of in- 

 tense light and dryness approaches some 

 of the pairs of leaflets fold together, 

 slightly reducing the surface exposure. 

 As the unfavorable condition continues, 

 more leaflets fold together, then still 

 others, until finally all the leaflets may be 

 folded together, and the leaves them- 

 selves may bend against the stem. It is 

 like a sailing vessel gradually taking in 

 sail as a storm approaches, until finally 

 nothing is exposed, and the vessel weath- 

 ers the storm by presenting only bare 

 poles. These are but a few illustrations 

 of the very numerous devices for escap- 

 ing too intense light and the dangers 

 which accompany it. 



One common danger in temperate re- 

 gions comes from the lowering of the 

 temperature each night, which sometimes 

 may chill the living substances to the 

 danger point. This is particularly dan- 

 gerous to seedlings, whose tender struc- 

 tures have not vet developed the ordi- 



