piration or breathing; the taking in of 

 oxygen and the giving off of carbon di- 

 oxide. However, the amount of poison- 

 ous material rejected by the plant is 

 small, not sufficient to counter-balance 

 the good that they do in purifying the 

 air. In any work done by a living or- 

 ganism, oxygen is needed to supply the 

 motive power,, and plants, of course, are 

 no exception to the rule. Respiration 

 takes place day and night, but the other 

 two processes, food manufacture and 

 transpiration, only in the day time. One 

 can see how important air and sunlight 

 are to the plant, and how necessary 

 the devices for securing both. 



The petiole holds the leaf out from 

 the stem, while the arrangement of the 

 leaves themselves on the stem prevents 

 as much as possible the interference of 

 one with the other. Some leaves are 

 arrayed in two flat rows on each side of 



the branch ; others are spirally placed ; 

 others have longer petioles on the lower 

 part of the plant than on the upper end, 

 so overlap or extend beyond each other. 

 Leaves protect themselves in various 

 ways from extremes of heat and cold 

 and from too much water. The mullein 

 leaf is covered with downy hairs, the ivy 

 is smooth, and the orange and lemon 

 leaves waxed, so as to shed the water. 

 During the heated part of the day leaves 

 often droop or fold their leaves together, 

 as does the oxalis, so that less moisture is 

 lost by transpiration. Many plants at 

 night assume a different position from 

 that of the daytime, thus protecting 

 themselves from chill. A great many 

 other schemes for securing the best pos- 

 sible results are resorted to by leaves, 

 some of w^hich we may find out by close- 

 ly watching their habits. 



PART ni, ROOTS 



Oliver Wendell Holmes spoke of trees 

 as tails waving in the air, while the real 

 body of the plant, the roots, lived be- 

 neath the ground. It is true that roots 

 often exceed in length that part of the 

 plant which is above ground, and that 

 they perform for the plant a very neces- 

 sary work. Indeed, there are three 

 pieces of work that the root has to do 

 that are essential to the welfare of the 

 plant, the first of which is to fix it in 

 place. 



One of the chief differences between 

 plants and animals is that plants do not 

 .move about. They move: stems twine, 

 climb, and bend toward the light, as we 

 readily see, but they do not move from 

 place to place. Some plants almost do 

 away .with this distinction when they 

 bend down, take root, and form new 

 plants, as in the case of the walk- 

 ing fern ; yet , after all, it is never 

 detached from the soil. In water 

 plants where the plants are sub- 

 merged, this work of attaching is the 

 main work of the root, for all parts of 

 the plant are adapted for absorption, an- 

 other function of roots. 



The primary root of a seedling, where 

 it develops from the first stem, grows 

 downward and commonly begins to 



branch. However, this first root some- 

 times gets the start of the branches and 

 we have one distinct- root, called a tap 

 root. This is the case with the parsnip 

 and radish, where the branches are in- 

 significant. The oak has an immense 

 tap root which fixes it so firmly that it 

 can weather the fiercest storm. But 

 oftener the main root is not distinct 

 from the other branches. 



In land plants the absorption of moist- 

 ure from the earth and of salts in solu- 

 tion is of first importance to them. 

 We have seen that a great deal of moist- 

 ure is lost daily by the plant through 

 the leaves. If there we~e not some way 

 of replacing at least a part of this moist- 

 ure, the plant would become flaccid and 

 and wilted. This is the second work of 

 roots ; and they are well fitted to per- 

 form it. The first essential is as large 

 a surface expansion as possible. 



We seldom realize how much area is 

 covered by roots. If we^dig up a plant, 

 we oftener break it off near the surface 

 than get the whole root. Near the tip 

 of the root fine hairs are produced, 

 called root-hairs : these increase the ex- 

 pansion and aid greatly in absorption. 

 These hairs adhere very closely to par- 

 ticles of earth and are able to extract 



66 



