62 PHYSIOLOGY OF THE LEAP. 



open them at another ; ahnost every garden contains some plants in 

 which this phenomenon may be observed ; it is particularly remark- 

 able in the sensitive plant, and the tamarind-tree. This folding up 

 of the leaves at particular periods, has been termed the sleep of plants ; 

 a celebrated botanist,* remarks, " this may be as useful to the vege- 

 table constitution, as real sleep to the animal." Linnaeus was led to 

 observe the appearance of plants in the night, from a circumstance 

 which occurred in raising the Lotus plant ; he found one morning 

 some very thrifty flowers, but on looking for them at night, they 

 were no longer visible. This excited his attention, and he began 

 to watch their unfolding. He was thus led to investigate the appear- 

 ance of other plants at the same time, and to observe their ditferent 

 manner of sleeping. He found, as darkness approached, that some 

 folded their leaves together, others threw them back upon their pe- 

 tioles, or closed their corollas, thus exhibiting a variety of interest- 

 ing phenomena. This state of relaxation and repose seems to depend 

 on the absence of light ; with the first rays of the morning sun, the 

 leaves recommence Iheir chemical labours by drawing in oxygen, 

 the fibres of the roots begin to imbibe sustenance from the earth, and 

 the whole vegetable machinery is again set in motion. It is not so- 

 lar light alone which seems capable of producing its elfect on plants; 

 this has been proved by the following experiment. A botanist placed 

 the sensitive plant in a dark cave, and at midnight lighted it up with 

 lamps ; the leaves which were folded up, suddenly expanded ; and 

 when, at midday, the lights were extinguished, they again as sud- 

 denly closed. 



Falling of the Leaf. 



The period at which leaves fall is termed the Defoliation] of the 

 plant. The " fall of the leaf" may be referred to two causes ; the 

 death of the leaf, and the vital action of the parts to which it is at- 

 tached. If a whole tree be killed by lightning, or any sudden cause, 

 the leaves will adhere to the dead branches, because the latter have 

 not the energy to cast them off. The development of buds, the 

 hardening of the bark, and the formation of wood, accelerate the fall 

 of the leaf. Heat, drought, frosts, wind, and storms, are all agents in 

 their destruction. 



About the middle of autumn, the leaves of the Sumac and Grape- 

 vine begin to look red, those of the Walnut, brown, those of the 

 Honeysuckle, blue, and those of the Poplar, yellow ; but all sooner 

 or later take that uniform and sad hue, called the dead-leaf colour. 

 The rich autumnal scenery of American forests is regarded by the 

 European traveller with astonishment and delight, as far exceeding 

 any thing of the kind which the old world presents. Painters, who 

 have attempted to imitate the .splendid hues of our forests, have, by 

 foreigners, been accused of exaggeration ; but no gorgeous colour- 

 ing of art can exceed the bright scarlet, the deep crimson, the rich 

 yellow, and the dark brown, which these scenes present. 



After what you have now learned of the anatomy and physiology 

 of leaves, you will probably be induced to pay attention to them in 

 their different stages ; fi'om their situation in the bud, to their full 

 growth and perfection ; you will feel a new interest in their change 

 of colour, nov/ that you understand something of the philosophy of 

 this change ; — even the dry skeletons of leaves, which the blasts of 

 autumn strew around you, may not only afford a direct moral lesson, 



♦ Sir .T. E. Smith. 



t From de, signifying to deprive of, and folium, Icnf. 



Sleep of plants— Cause of the sleep of plants— Defoliation— Fall of the leaf. 



