VEGETABLE PHYSIOLOGY. 25J 



bie, varying with the seasons, both in mechanical mixture, 

 and chemical composition. 



Membrane, and Fibre. The solid parts of Plants are com- 

 posed of vegetable Membrane, and vegetable Fibre. Of these 

 are formed the common organic structures, the Cellular, and 

 Vascular tissues, organizations which will be explained here- 

 after. 



Vegetable Membrane. This is a thin, transparent, colorless 

 film, composed of minute organic fibres, arranged parallel to 

 each other, and united by a glutinous substance. In its sim- 

 plest state, it forms the sides of the cells of Plants, but when 

 more condensed, it composes the sap vessels, and the general 

 covering, or outer skin of vegetables. 



Vegetable Fibre. This is most evident in the spiral ves- 

 sels of Plants, which, as we shall see directly, are composed 

 of one or more threads, twisted spirally, so as to form a cy 

 lindrical tube. 



Cellulur Tissue. The interior of all Plants, when exam 

 ined with a microscope, appears composed of minute cells, 

 which are lined with the membrane above described. The 

 best method of examining this structure is to put a thin trans- 

 verse slice of a stem in a drop of water, and place it under 

 the magnifier. 



The simplest form of these cells is globu- Fig. 221 



lar, as represented by Fig. 221, which is 

 a transverse section of the stem of Nastur- 

 tion, (Tropoeolum majus,) highly magnified. 

 The sizes of these cells differ greatly in 

 different Plants, and in different parts of the 

 same Plant. Kieser states that the diame- 

 ter of each individual cell varies from the 

 55th to the 330th part of an inch ; so that 

 from 3000 to 100,000 cells would be con- 

 tained within the space of a square inch of. surface. 



In their original state, or in the young Plant, these vesi 

 cles, as above stated, are of a globular form ; but they are 

 soon transformed into other shapes, either by the mutual com- 

 pression which they sustain by being crowded into a limited 

 space, by the growth of the Plant, or from unequal expansion 

 in the progress of their own development. 



From the first of these causes they assume the form ot 

 hexahedral or six-sided figures, or double six-sided pyramids 



