LESSON 4.] STRUCTURE OP VEGETABLE TISSUES. 11 



58. Other tissues, which are peculiar to animals, cannot be re- 

 ferred to the same origin ; but these will be found to have a grade 

 of organization even lower than that of simple isolated cells, and to 

 be referrible to the solidification of the plastic or organizable fluid 

 prepared by the assimilating cells, and set free by their rupture. 



59. We shall find, however, that (as in plants) all the tissues 

 most actively concerned in the vital operations, retain their original 

 cellular form ; and it will be easy to refer to distinct groups of cells 

 in the bodies of animals, not merely for the performance of the func- 

 tions of Absorption, Assimilation, Respiration, Secretion, and Re- 

 production, which are common to them with plants, but also those of 

 Muscular contraction, and Nervous action, which they alone per- 

 form. 



LESSOR IV. 



ON THE STRUCTURE OF VEGETABLE TISSUES. 



60. All the tissues of plants are remarkable for their simplicity 

 of structure, as compared with the tissues of animals. 



61. In their earliest and simplest condition, plants are found to 

 consist of a series of minute vesicles, composed of a membrane called 

 cellulose. 



62. It is also known as simple membrane, and possesses char- 

 acteristics which distinguish it from all other tissues, that is to say ; 

 it is elastic, transparent, easily permeable by fluids, and structureless. 



63. It constitutes the basis of all vegetable tissues, and it is uni- 

 versally present. With regard to its chemical characters it appears 

 to be closely allied to starch ; treated with sulphuric acid, it turns 

 yellow ; when, if subjected to the action of tincture of iodine, a beau- 

 tiful purple color results, indicating the iodide of starch. 



64. As the higher plants advance in growth, they are found to 

 consist of two kinds of tissue cellular and vascular, and these are 

 variously modified to form the elementary organs. 



65. By their union, the elementary tissues form the compound 

 organs, by which the several functions of plants are carried on. 



CELLULAR TISSUE. 



66. By the conjunction of a series of minute vesicles, utricles, 

 or cells, this tissue is formed. They appear to be perfectly spherical 



