1 7fi PHYSIOLOGICAL BOTANY. PART II. 



carbon is now fixed under the form of a nutritive ma- 

 terial, which is conveyed back into the system ; and this 

 material is further elaborated for the development of all 

 parts of the structure, and for the preparation of certain 

 secreted matters, which are either retained within or 

 ejected from the plant. These several processes may 

 be designated : 1 . Absorption ; 2. Progression of sap ; 

 .'{. Kxhalation ; 4. Respiration; 5. Retrogression of 

 proper juice ; 6. Secretion ; 7- Assimilation. 



FIRST PERIOD OF NUTRITION. 



(l6'0.) Abtorption. That plants absorb moisture 

 from the soil in which they grow admits of easy proof. 

 The extremities of the fibres in which their roots ter- 

 minate, are not covered with an epidermis like the rest 

 of the surface, and consequently the cellular texture is 

 there exposed, and constitutes the " spongiole," or true 

 absorbing organ. As plants do not possess the power 

 of locomotion, it is essential that their food should be so 

 universally distributed that they may run no risk of 

 perishing from want of a constant supply. It is further 

 requisite that their food should be offered them in a 

 fluid form ; for it is an established principle in ve- 

 getable physiology, that the spongioles are incapable 

 of absorbing any matter in a solid state. Whatever 

 therefore, is to be received into the system for the pur- 

 pose of nutrition must be held in a state of solution 

 in water. The three most important ingredients to be 

 found among the products of vegetation, are oxygen, 

 hydrogen, and carbon (see art. 14.) ; the two former are 

 the elements of water, and the third is an element of 

 carbonic acid, a gas which is every where present in the 

 atmosphere, and which may be detected in almost all 

 springs and other waters on the surface of the earth. 

 Water, again, in a state of suspension in the air, is also 

 present every where. Plants, therefore, receive a constant 

 supply of these three elements wherever they are placed 

 on the surface of the earth, in situations adapted to their 



