CHAPTER XXI 

 GEOWTH 



Definition. — It is not easy to say exactly what we mean by 

 the word growth. It does not denote simple increase in size 

 or change in shape. A dead, dry bean-seed will swell when 

 supplied with water; but if the dead bean-seed be dried 

 once more, the bean shrinks to its former dimensions. If we 

 supply water to a living bean-seed the latter does not merely 

 become larger ; a great change in the form ensues. The little 

 embryo develops roots and stem and leaves. We know 

 perfectly well that if we take away the water from the bean- 

 seedling we cannot once more change it into a bean-seed. 

 Growth is a permanent change in the form of the plant, and 

 can take place only in living parts. 



CONDITIONS ESSENTIAL TO GROWTH. 



(i.) The plant must be living, (ii.) Only parts of a plant 

 which are in a youthful (embryonic) condition, or can be 

 brought into a youthful condition, are capable of growing. 

 This statement can be proved only by the aid of the compound 

 microscope, but it may be illustrated by examples. We 

 know, for instance, that we continue to grow in height only up 

 to a certain age. Again, the young growing points of roots 

 and stems are the only parts of them which grow in length. 

 In the grasses there is an apparent exception to this last 

 statement, for the tissue just above the old nodes can grow ; 

 but this tissue really remains in a " youthful condition " in 

 spite of age. (iii.) A supply of water is essential. Water is 

 required, first, because it is a constituent of the living substance. 

 Furthermore, it is a food-substance. In the third place, it serves 

 to carry the nutrient substances to the growing parts. Finally, 

 it keeps the fresh green parts of plants in a stiff and extended 

 condition, as is proved by observing the process of fading or 

 withering of drying parts of plants, (iv.) Oxygen is required 



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