LEAVES 



23 



amount of water given off by a few leaves is noted, some 

 vague idea may be formed as to the amount given off by a 

 great mass of vegetation, such as a meadow or a forest. 

 One observer has 

 stated that a single 

 stalk of corn during 

 its life (173 days) 

 transpired about four 

 gallons of water; and 

 that a single hemp 

 plant (140 days) 

 transpired nearly 

 eight gallons. An- 

 other observer esti- 

 mated that a sun- 

 flower, whose leaf 

 surface was approxi- 

 mately nine square 

 yards, gave off near- 

 ly one quart of wa- 

 ter in a single day. 



16. Growth. In 

 very young leaves 

 growth takes place 

 at the apex, but this 

 may cease early. The 

 subsequent growth 

 often occurs at the base of the blade, in a special growing 

 region, as may be seen in long and narrow leaves such as 

 those of grasses. To discover these special regions of 

 growth in leaves, some rapidly growing plants (such as 

 the gourds) should be cultivated in pots. When the 

 young leaves first appear, a scale should be marked off in 

 India ink with a pointed camel's hair brush on the petiole 

 (if there be one) and the midrib. The scale should be made 

 3 



FIG. 17. Transpiration experiment : a potted gera- 

 nium sealed with a rubber cloth and covered 

 with a bell jar ; the mist and droplets of water 

 on the glass more or less obscure the plant. 



