20 A HALF-HOUR WITH THE 



observe the little round disks with a black dot 

 in the middle, is to make a thin longitudinal section 

 of a piece of deal, and place it under a half or 

 quarter-inch object-glass, when they will be readily 

 apparent. The application of a drop of water on 

 the slide, or immersing them in Canada balsam, 

 will bring out their structure better. 



If we take the leaf-stalk of a strawberry or of 

 garden rhubarb, and make a transverse section all 

 round, nearly to the centre of the stalk, the lower 

 part will at last break off, but be still held to the 

 upper by very delicate threads. If we examine 

 these threads, we shall find that they are fibres 

 (Fig. 43, PI. 2), which have been left by the break- 

 ing of the vessel in which they were contained. 

 These vessels are called " spiral vessels," and are 

 found in the stems and leaves of many plants 

 (Fig. 42, PL 2). Sometimes these vessels are found 

 branched, as in the common chickweed (Fig. 45, 

 PL 2). Occasionally the spiral fibre breaks, or is 

 absorbed at certain points, leaving only a circular 

 portion in the form of a ring (Fig. 44, PL 2). 

 Such vessels are called " annular," and may be 

 observed in the roots of growing wheat ; also in 

 the leaves of the garden rhubarb. A modification 

 of this kind of tissue is seen in the stems and roots 

 of ferns, in which the vessel assumes a many-sided 

 form (Fig. 47, PL 2). Sometimes the spiral fibre 

 is free (Fig. 240, PL 8). 



The bark as well as the wood of trees affords the 

 same appearance under the Microscope. If a piece 

 of the bark of any plant be examined by means of 

 a very thin transparent section, and placed upon a 

 slide, and put under an inch or a half-inch object- 

 glass, the structure of the bark may be easily seen. 

 On the outside of all is the cuticle, or epidermis, 

 and under this lie two layers, composed, like the 



