CHAPTER II 



plants will grow to maturity, whereas seedlings grown in pure water 

 perish soon after the food in the seed is exhausted. Further experi- 

 ments show that the presence of iron is necessary to the formation of 

 chlorophyll ; and it is well known that nitrogen is a constituent of 

 protoplasm. Now, although nitrogen forms a large percentage of the 

 air, experiments seem to prove that it is of service to plants only 

 when it enters in combination with other substances. So the nitro- 

 gen required by plants is absorbed mainly from the ground, though 

 sometimes from the air, and it is in the form of nitrates or of com- 

 pounds of ammonia. Nitric acid and ammonia arise from the decay 

 and oxidation of dead organic matter, and since all putrefaction is 

 due to bacteria, we may say that nitrogen is prepared for the plants 

 by bacteria in the soil. 



Corn and castor bean seedlings are excellent for illustrating the 

 ascent of crude sap, but their own red pigment should be located 

 before the red ink is used. The natural pigment is in or near the 

 epidermal cells, where it is supposed by some botanists to serve as a 

 screen; the artificially colored fluid will always travel by way of the 

 woody tissue. Of course for higher work more accurate experiments 

 than this have been devised for showing this function of woody tissue. 

 The fact of the existence of this woody system, characteristic of 

 higher plants, is perfectly apparent in seedlings, but its full signifi- 

 cance and the reasons why lower plants can do without it, need not 

 be discussed here. 



The cellular structure of stems is far too complex a subject to be 

 treated fully in an elementary work, but it will be again referred to 

 in Chapter VIII. Drawing No. 2, Fig. 10, which is of course partly 

 diagramatic, is introduced here only to emphasize the division of 

 labor among different kinds of cells. This conductive tissue of wood 

 cells and vessels extends to the farthest tips of the leaves. The epi- 

 dermis of the leaves serves for protection and prevents excessive 

 evaporation. It has a cuticle that is almost impervious to water. 

 The moisture that is given off by leaves escapes mainly through the 

 pores or stomata, and the two guard cells of the stomata, by separat- 

 ing or closing, regulate the amount of transpiration. The typical 

 horizontal leaf has, within the epidermis, palisade cells on the upper 

 side and spongy tissue beneath; that is, the upper cells are little 

 cylinders compactly arranged and at right angles with the epidermis; 

 below, the cells are so loosely arranged that there are many air pas- 

 sages that communicate with the outside world through the stomata. 

 These intercellular canals also convey the moisture that is evaporated 

 from the cells. 



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