OSMOSIS 7 



take up nitrogen from air, and as these bacteria die in the soil, they 

 thus constantly leave it richer in nitrogen. Bacteria of this char- 

 acter are associated with legumes, as clover or peas, so that the grow- 

 ing of clover always leaves the land rich in nitrogen. 



Thus, four of the important elements of plants come from the 

 air oxygen, hydrogen, nitrogen, and carbon and six from the soil. 

 (Nitrogen, however, is first combined in the soil.) 



Relative Composition of Plants. If 100 pounds of green corn 

 plants or grass be thoroughly dried, about 80 pounds of weight will 

 be lost, and only 20 pounds of dry matter (Fig. 1 G ) remain. Air- 

 dry seeds or hay contain from 10 to 15 per cent water. 



If the dry matter is now thoroughly burned, there will be left 

 about one pound of ash. The 19 pounds that went up in the fire 

 represent what came from air, while the one pound of ash is all that 

 came from the soil. The ash represents about one per cent of green 

 plants or four per cent of dry matter. 



HOW ROOTS AND LEAVES PERFORM THEIR FUNCTIONS 



The Root System. The functions of roots are to secure water 

 and plant food elements from the soil, acting also as an anchor. The 

 root system is usually much more extensive than commonly sup- 

 posed. With wheat or oats, roots penetrate two to four feet deep, 

 being deeper on well- drained, porous soils than on compact or wet 

 soil. The lateral spread is usually greater than the depth, especially 

 with intertilled crops, as corn or potatoes. Corn roots frequently 

 spread four to six feet laterally. 



New branch roots are constantly produced, so long as the plant 

 is growing. It is only the small new roots that have organs, called 

 root-hairs, for absorbing water directly from the soil. 



Root-hairs. The root-hairs are very small, single-celled organs, 

 produced in a zone near the tip of the new roots (Fig. 2). They 

 function for only a short time, then die as the root extends, and new 

 root-hairs are produced near the tip. Water is not absorbed by the 

 roots, bnt only by the root-hairs. The root-hairs absorb soil water by 

 a process known as osmosis. 



Osmosis. The sap of the root-hairs being denser than the soil 

 water, the denser solution absorbs the weaker. This principle can 

 be demonstrated with a slice of potato or apple. Put a tablespoonful 



