CH. xii PHYSIOLOGY OF GROWTH AND MOVEMENT 153 



the means by which the green parts of plants are kept fresh, and 

 the cells turgid. 



3. Oxygen. Those parts of the plants where growth is 

 proceeding require oxygen, for without it no energy can be 

 produced. No growth can take place without energy. Energy 

 is produced when a plant respires (p. 125). 



4. Food Materials. There must be suitable food materials 

 present. Food may either be stored up in seeds, or it may be 

 taken along with water from the soil, or be obtained by the 

 leaves. 



5. Cells in an Embryonic Condition. The cells of some 

 parts of the plant must be in such a condition that they can 

 divide and increase in size. 



6. Light. While light is not absolutely essential for growth, 

 it is still necessary for healthy growth. Plants will grow faster 

 in the dark than in the light, as is well seen in the case of 

 Rhubarb, which when forced in the dark, has small leaves and 

 long and slender stems. When grown in the light the stem of 

 Rhubarb is short and thick, and its leaves large. Speaking 

 generally, it may be said that most plants grown in the dark 

 have soft stems, which are very much elongated, and of a pale 

 colour. The leaf-blades, in similar circumstances, are small, 

 and yellow in colour, and the tissues of the plants have thinner 

 walls, and contain more water than those grown in the light, or, 

 light may be said to retard growth. 



If wheat seeds are sown too close together so that light cannot 

 pass between the plants, the stems become long and so slender 

 that they can no longer support the ears of corn, and the stems 

 bend under the weight. This constitutes the so-called laying of 

 wheat. It can be prevented by leaving a sufficient space between 

 the rows to enable light to pass between the plants, when the 

 growth will be normal. 



The rapid growth of shoots produced from bulbs, tubers, 

 rhizomes, and seeds is especially valuable, for the light is thus 

 reached very quickly, and the plants are then capable of in- 

 dependent nutrition. 



Plants grow more rapidly during the night than day. During 

 the day assimilation goes on and the materials then stored are 

 used up during darkness in producing a permanent change in 

 the plant. 



