26 A HALF-HOUR WITH THE 



seen with several branches, giving them a stellated 

 appearance (Fig. 79, PL 3). These hairs may be ex- 

 amined as opaque or transparent objects, when im- 

 mersed in a little glycerine. The hair of the tobacco- 

 plant presents a peculiar knobbed appearance (Fig. 

 77, PL 3). The presence of these hairs is said to be 

 the test of the purity of tobacco. Sometimes hairs 

 are covered over with little dots, which are supposed 

 to be deposited after the growth of the cells of the 

 hair. Such hairs may be seen in the common ver- 

 bena (Fig. 80, PL 3). Occasionally an elevation, 

 consisting of several cells, is formed at the base of 

 a hair (Fig. 81, PL 3). When these cells contain 

 a poisonous secretion, which is transmitted along the 

 tube of the hair, the hair is called a glandular hair, 

 or sting. Such are the hairs of the common sting- 

 ing-nettle (Fig. 82, PL 3). 



The hairs constituting the down or " pappus " of 

 compositous plants assume a variety of forms. The 

 seed or fruit of the common groundsel has a beau- 

 tiful crown (Fig. 235, PL 8). The pappus of the 

 dandelion appears notched (Fig. 236). The burdock 

 has a cottony hair (Fig. 237), while the goats- 

 beard is like a feather (Fig. 238). 



If a hair is examined in its growing state, with 

 an object-glass of one quarter of an inch focus, a 

 movement of the particles in its interior is often 

 observed. This is easily seen in the hairs around 

 the stamens of the common Spiderwort (Tra- 

 descantia Virginica). Such movements are very 

 common in the cells of water-plants. One of 

 those most commonly cultivated in aquaria at the 

 present day, the Valisneria spiralis, affords the best 

 example of this interesting phenomenon. In order 

 to observe this movement, a growing leaf of the 

 valisneria should be taken, and a longitudinal slice 

 should be removed from its surface, by means of a 



