14 POPTOAR ILLUSTRATIONS OF 



and their use, like our own blood vessels, is to convey the fluids 

 taken up by the roots to all parts of the leaf where important 

 changes are effected, and to return it so altered as to subserve the 

 purpose of nutrition to the plant. These vessels have all more or 

 less a spiral character, like those shown in Fig. 5, e, p. 13. Some of 

 them carry air, others fluid. In our wheat-plant these vascular vessels 

 stop short in their development at a certain stage. In other plants, 

 as most of our trees, new matter is continually being added to them 

 externally, so that they ultimately form hard tissue, known as wood. 

 There is also another system of vessels in leaves called "laticiferous," 

 which, in fact, constituted the "proper vessels" of old writers. 

 They are principally connected with the bark of trees. They convey 

 a fluid called "latex," red, white, yellow, or colourless. They 

 divide into very minute branches, only discovered by the highest 

 power of the microscope. It is from these vessels that the milky 

 latex exudes when we tear across the leaf of a lettuce. Leaves of 

 course present an infinite variety, and their study is one of absorbing 

 interest to the botanist. 



I have said enough, however, to give a general view of the 

 subject, and to allow us to take an intelligible glance at the 

 functions of leaves. 



We have seen that the roots, by means of their spongioles, take 

 up fluid from the soil containing certain mineral matters in solution. 

 This fluid is chemically altered, and has added to it organic matter 

 taken in by the leaf from the atmosphere, and it then becomes the 

 fluid known as sap. In other words, it is the nutritious fluid of the 

 plant, bearing to it the same relation as our own blood does to our 

 bodies. 



I have shown also that the protophyton absorbs carbonic acid 

 from the atmosphere, and it is by this means that the great amount 

 of charcoal contained in the vegetable growth of the earth is pro- 

 duced. It is altogether a mistake, therefore, to assume that plants 

 are injurious to health if grown in our living apartments. Carbonic 

 acid in excess in our atmosphere is injurious to animal life. Plants 

 remove it. It is to this power of absorbing and appropriating to its 

 use the carbon of the atmosphere that we are indebted for our 

 coal, gas, and fire. It is this which gives the essential element to 

 the meat produced by our flocks of oxen and sheep. It is, in fact, 

 a source of infinite blessing and comfort and happiness to the whole 

 human race. Leaves are the great manufacturers of food in every 

 part of the world. If we search the highest mountain, penetrate 

 into primeval forests, or go down into the depths of the sea, we 

 shall find leaves, or modifications of leaves, ever at work night and 

 day in preparing the means by which animal life is supported. The 



