2i6 General Botany 



the branch, the apple will be short, flat, and vertically elliptical 

 instead of round in cross-section. If it hangs obliquely from the 

 branch, the fruit will be obliquely elongated. In all cases the 

 apple is longest in the direction of the pull of gravity. 



The annual rings in the horizontal branches of trees are often 

 thicker on the lower side than on the upper. When corn and 

 other grasses are blown down by wind, they again become up- 

 right, because gravity not only stimulates growth of the nodes, 

 but causes the lower half to grow faster than the upper half, until 

 the stem is brought again to a vertical position. 



Chemical elements essential to plants. We have already 

 learned that in order to have a plant grow the soil must furnish 

 it with sufficient water for transpiration and for the manufacture 

 of food. At the same time, the soil must not be so filled with 

 water as to exclude oxygen from the roots. Carbon dioxide and 

 water supply the plant with the three elements, carbon, hydrogen, 

 and oxygen, needed in building carbohydrates. 



From the soil solution plants obtain other essential chemical 

 elements used directly, or indirectly, in the manufacture of food 

 and in the development of their tissues. These elements are 

 potassium, calcium, magnesium, nitrogen, phosphorus, sulfur, iron, 

 and possibly manganese. The growth of plants is hindered, and 

 certain plants are excluded from soils that contain insufficient 

 amounts of any of these substances. It should be borne in mind, 

 however, that from 60 to 95 per cent of a plant is water and that 

 most of the remainder is organic matter. When plants are 

 burned, the water and organic matter pass into the air and only 

 the mineral matter remains as ash. The ash seldom amounts to 

 more than 3 per cent of the green weight, and sometimes it is as 

 low as .3 of I per cent. It is evident, then, that each of these 

 essential elements has one, or several, special uses in the plant, 

 and no other can be successfully substituted for it. There are 

 other elements, like silicon, aluminium, and sodium, that accumu- 

 late in plants but take no necessary part in either their processes 



