74 



Vegetable Physiology. 



parts of stems, especially of pith, such as that of the elder, where 

 they are very large, or in the pith of rushes, the substance used 

 for the wicks of rushlig^hts. Slices of these structures look like 

 pieces of network under the microscope, and might mislead a 

 person glancing at them hastily, but the deception is readily 

 detected ; it depends upon our seeing only part of the bags (the 

 sides) at a time, just as the joints in a piece of brickwork appear 

 as a network of lines upon the surface of a wall. 



We cannot, indeed, better illustrate the mode in which vege- 

 table structure is made up than by comparing it with brickwork, 

 the single bricks being represented in the plant by the little bags 

 before-mentioned. We can imagine bricks to be of any shape, 

 such as oblong, square, flat like tiles, &c., and then they may be 

 packed close together ; sometimes the structure of vegetables is 

 of this form and arrangement, as especially in hark. If the 



Fis:. 1. 



Slice of the bark of a j'oung branch of Beech, magnifled 200 diameters. 



bricks were made round or oval, however, they could not be 

 packed so as to touch at all points, but would leave passages be- 

 tween them, just as is the case when a number of cannon balls 

 are piled together ; the annexed drawings will represent the way 

 in which the loose and spongy textures of plants are formed. 



Fig. 2. 



Fragment of a cross slice from the stem of the 'Wliite Lily, magnifled 200 diameters. 



We have spoken ot the minute parts of which substance is 



