PROTECTION 



243 



time are called deciduous. Deciduous plants are not 

 confined to temperate regions; wherever there are long 

 dry seasons many plants drop their leaves at the beginning 

 of the dry season, just as with us they drop them at the 

 approach of winter. Evidently the effect of dry weather 

 upon the fall of leaves is about the same as the efiect of 

 cold weather. 



It should not be understood that evergreen plants keep 

 the same leaves throughout life. They too shed their 

 leaves, but gradually rather than all at about the same 

 time ; new leaves appear at about the same rate that the 

 old ones fall The life of a leaf, save in some annuals, is 

 much shorter than the life of the plant which bears it. 

 The cotyledons and the leaves of some plants which grow 

 under very dry conditions may endure only a few weeks 

 or even days. Evergreens rarely retain any one leaf 

 more than a year or two, though the needle-Uke leaves of 

 some pines have been observed to endure as long as ten 

 years. The Hfe of most leaves is hmited to a few months, 

 varying as the season of active work is long or short. 



Why is it that leaves cannot do their work as well in 

 winter as in summer? Evidently it is not on account of 

 lack of light, for the sun often shines as brightly on winter 

 days as on summer days. It is not on account of lack of 

 carbon dioxide, for carbon dioxide is always present in the 

 air. Water too appears to be abundant. On account of 

 the cold, however, the water, especially the water in the 

 soil, is more likely to be soHd than to be liquid. Herein 

 appears to be a principal reason why cold weather arrests 

 the work of plants. Water in the form of ice is of no more 

 use to the plant than no water at all. Only in liquid form 

 can water enter the roots or move through the plant body 



