388 Comparative Physiology. 



porous substances in the soil, as pieces of charcoal, bones, decay- 

 ing wood, porous stones, &c. One of the best materials for 

 encouraging the formation of fibres is rotted leaves, or the char- 

 coal from the half-smothered combustion of the spray of young 

 shoots, leaves, &c. : the leaves and young shoots contain all the 

 substances necessary for keeping vitality active ; if the heat is 

 violent it may dissipate the ammonia and fibrine of the shoots, 

 and do harm. The light and heat of the sunbeam are also 

 necessary to produce the phenomena of endosmose, without 

 which absorption and circulation could not go on ; electricity 

 also will probably assist in the vital activity or irritability of 

 the tissue, which increases circulation and absorption. To assist 

 aeration, heat and light are necessary to produce exhalation ; to 

 promote the interchange of gases, it is necessary that the leaves be 

 kept free of dust and extraneous matter ; and when the state of 

 weather out of doors, or confinement in houses, has clogged the 

 leaves, the lungs of plants, it is absolutely necessary to syringe 

 and keep them clean. The important organic chemical action 

 of the leaves, so necessary to prepare the food for assimilation, 

 is also promoted by the same means. Into the functions of 

 the preparation of the food (or digestion), absorption, circula- 

 tion, and aeration, science has thus enabled us to obtain so much, 

 insight as greatly to facilitate and augment their action by proper 

 cultivation. When I come to treat of each of these functions 

 in a special manner, the subject will be rendered more intel- 

 ligible than it could be in a general outline. 



Of the remaining functions of assimilation, secretion, and 

 reproduction, we are more ignorant ; of that mysterious power, 

 which from the simple membrane of the organs (to all micro- 

 scopical and chemical observations seeming alike) can elicit so 

 many and such varied products, we are completely ignorant. 

 Chemical observation of the secreted products, however, and 

 analysis of particular plants, enable us to know what food it is 

 necessary to provide for each ; immense additions have lately 

 been, and are still being, made by chemists to our information 

 on these subjects, and great benefit should redound to practice 

 therefrom. Of the same mysterious vital power, which from 

 the rudiments of a branch can prepare such seemingly difierent 

 products as the parts of the flower, and can change the repro- 

 ductive bud into the more perfect though more changeable 

 organ the seed, we know also comparatively little. Observ- 

 ation, however, has established that a duly elaborated state of 

 the food is essential ; and that where light and heat, or the 

 influence of the sunbeam, cannot be got in sufficient quantity, 

 we can assist the operations of nature by lessening the quantity 

 of sap to be elaborated, and produce the necessary elaborated 

 state for fructification from a small quantity, which our insuf- 

 ficient means will not enable us to do from a larger. By 



