44 THF LEAF. 



fact that if apples and pears in a green state be placed in 

 tho dark, they do not ripen under any degree of heat, but 

 soon decay. 



Absorption of Fluids. With respect to the absorption of 

 fluids by the leaves of plants, and their emission of fluids, or 

 perspiration, experiments have determined that this office is 

 performed chiefly by their under surfaces. It is well known 

 that when a single branch of a plant is placed in a vessel 

 of water during a dry season, the whole plant will revive 

 and continue to grow, though no moisture be given to the 

 root. If, however, the under surfaces of the leaves be first 

 covered with oil or varnish, so that no water can be absorbed 

 by these parts, no such effect will be produced, and the herb 

 will wither, as though no water had been applied. Again, if 

 a green leaf be plucked, and laid with its upper surface on 

 the water, it will wither nearly as soon as if no water 

 touched it. But if it be placed with its under surface on the 

 water it will remain green for weeks. 



Irritability of Plants. The irritability of plants, as indica- 

 ted by their leaves, and sometimes by their flowers, also pre- 

 sents a subject of curiosity and interest. ]VI an y flowers col- 

 lapse, or cl^se their petals during the night, or when carried 

 into a dark place during the day. Some are so sensitive as 

 to begin to collapse whenever the sun is obscured by a cloud, 

 even for only a few minutes, and expand again as suddenly, 

 when the light becomes strong. This collapse is called the 

 sleep of plants. 



The Sensitive Plant (Mimosa sensitiva,) possesses this prop- 

 erty to an uncommon 'degree. Its branches, or leaves, on 

 being touched with the finger, instantly begin to retire. The 

 branch drops down from its erect position, while its leaves 

 fold themselves closely together. The same effect is produ- 

 ced on withdrawing the light, or during the darkness of night. 

 It is said that a highly interesting effect is produced on con- 

 veying a strong light into a room containing Sensitive Plants, 

 during the night. They soon begin to indicate by their mo- 

 tions, that they are disturbed in their sleep, and as they awake, 

 their branches and leaves, from being in a collapsed and tor- 

 pid state, become erect, as though sensible of the presence 



Whru part of the leaf performs the office of absorption and emission of 

 fluids ? How is it proved that these offices are performed by the under 

 surface of the leaf? What is meant by the irritability of plants ? What 

 plant is D^culiarly irritable ? What is said to be tho effect of carrying a 

 strong light into a dark room containing sensitive plants ? 



