64 PHYSIOLOGY OF THE LEAF. 



throiT^li the stomata ; these are only open in the j^i'esence 

 of lignt, therefore in the dark this process cannot procf^ecl. 

 Ahsorption is the ])eciiliar office of the root, but leaves also 

 perform this function, as water sprinkled on the leaves of a 

 parched plant is well known to revive it. Respiration is per- 

 formed chiefly by the leaves, but sometimes by the root ; it con- 

 sists in the absorption of oxygen and giving off carbonic acid. 

 Digestion consists in the decomposition of carbonic acid, the 

 fixation in the 23lant of solid carbon, and the evolution of 

 oxygen, 



The upper surface of leaves is usually of a deeper green, and performs a more 

 miportant part in respiration than the under surface. The upper surface also repels 

 moisture : a cabbage-leaf after a shower or lieavy dew, shows the moisture collect- 

 ed in drops, but with no appearance of its being absorbed by the leaf. It has been 

 found that the leaves of plants laid with their upper surfaces upon water, sooa 

 wither ; although the leaves of the same plants, placed with theu- under surfaces 

 upon water, retain their freshness for some days. Few among the vegetable tribes 

 are destitute either of leaves, or green stems, which answer as a substitute. The 

 Monotropa, or Indian pipe, is of pure white, resembling wax-work. Mushrooms 

 are also destitute of any green herbage. It is not known in what manner the de- 

 ficiency of leaves is made up to these vegetables. 



62. The period in which any species of plant unfolds its leaves, was called by 

 Linnffius, Frondcscence ; according to his observation in the climate of Sweden, the 

 opening of the leaf-buds of the birch-tree is the proper time for the sowing of 

 barley. Tlie American Indians held that the best time for planting Indian corn 

 was when the leaves of the white-oak first made their appearance ; or, according 

 to their expression, " are of the size of a squirrel's ears." 



a. One of the phenomena of leaves is their irritability, or power of contraction, 

 upon coming in contact with other substances. Compound leaves often possess this 

 property ; as the sensitive plant (mimosa sensitiva), and the American sensitive plant 

 (cassia nictitans) ; on bringing the hand near them they shrink as if from the touch 

 the warmth of tlie hand is supposed to produce the contractions of the leaves. 



b. The effect of light upon leaves is very apparent, plants being almost uniformly 

 found to present their upper surfaces to the side on which the greatest quantity of 

 light is to be found. 



c. Carbonic acid gas is the food of plants ; this consists of carbon and oxygen, 

 and is decomposed by the agency of light ; the carbon becomes incorporated with 

 the vegetable, forming the basis of its substance, while the oxygen is exhaled, or 

 thrown off into the atmosphere. 



d. M^y plants close their leaves at a certain period of the day, and open them at 

 another; almost every garden contains some plants in which this phenomenon may 

 be observed ; it is particularly remarkable 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 

 vegetable constitution, as real sleep to the animal." Linnaus was led to observe 

 the appearance of plants in the night, from a circumstance which occurred in raising 

 the Lotus plant ; lie found one morning some very thrifty flowers, but on lookhig 

 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 appearance of 

 other plants at the same time, and to observe their different manner of sleeping. 

 He found, as darkness approached, that some folded their leaves together, others 

 threw thein back upon their petioles, or closed their corollas, thus exliibiting a 



* Sir J. E. Smith. 



Of the upper and under surface of leaves. — 62. Frondescence — a. Irritability — b. EfTect of light — 

 c What efiecthas light upon the carbonic acid gas imbibed by plants? — d. Sleep of plant* 



