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CHAPTER THIRTEEN \^\ 



THE GROWTH AND FALL OF LEAVES ^^/.u — V V. 



We are all familiar with the fact that when a live seed is 

 planted in the soil it germinates, and that from it there devel- 

 ops a seedhng which continues to enlarge for a longer or shorter 

 time, depending on the plant and the conditions for growth. The 

 period of growth may be a month, as in the radish in midsummer, 

 or it may be hundreds of years, as in some trees. In the process 

 of growth vast quantities of food are consumed. During the 

 early stages of a plant's development most of the food it manu- 

 factures is used in this way. In order to grow, a plant must 

 make new protoplasm, develop new cell walls, and thicken and 

 strengthen old cell walls. Growth requires not only food but 

 energy as well. Indeed, a part of the energy derived from respira- 

 tion is used in certain chemical changes involved in growth. We 

 might expect assimilation, respiration, and food consumption to 

 be most active in young growing parts, and that this is the case 

 has many times been verified by experiment. Grow^th takes 

 place through the enlargement of cells already present in the 

 plants, through cell division, and through modification of cells 

 without enlargement. 



Conditions for growth. The conditions most favorable for 

 growth are abundant water and oxygen supphes and warm tem- 

 peratures, such as normally occur in summer. For the growth 

 of the plant as a whole, moderately strong hght is favorable 

 because it increases the supply of food. For the growth of leaves 

 in particular, medium light is generally most favorable. In 

 darkness the blades of many plants do not expand, and in very 

 intense hght they do not expand fully because of the retarding 

 effect of the hght itself and the excessive w^ater loss. 



The growing regions of leaves. By watching the development 

 of leaves on any common herb, or on the trees in spring, we can 

 see that growth takes place rapidly; also, that growth ceases 



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