LESSON 8.] SCLEROGEN, OR LIGNINE. 25 



teeth come in contact with a number of minute, hard particles in the 

 pulp cells ; they try to crush them with their teeth, but find them 

 much too dense. These constitute what is called the gritty tissue. 



151. A thin slice of a pear examined by the microscope (Fig. 37), 

 shows that this tissue is made up of a variable number of distinct 

 particles (a), which form a mass, of uncertain size, amidst the pulp 

 cells (b). 



FIG. 36. FIG. 37. 



Nut Ivory. Gritty tissue of the Pear. 



152. There can be no doubt that the intention of surrounding the 

 seeds with this peculiar tissue is, to afford them a certain degree of 

 protection ; and it appears to foreshadow the more perfect develop- 

 ment of a superior tissue designed for the same purpose (protection), 

 met with in the cherry, plum, peach, and other fruit stones. 



153. Some of the elements of the gritty tissue, of increased size, 

 are shown in the accompanying figure (Fig. 38) ; in common with 



FIG. 38. FIG 39. 



Sclerogenous elements of gritty tissue, magnified Testa of Nut Ivory. 



300 diameters. 



other like tissues, there is a central vesicle, associated with radiating 

 tubes. 



154. The testa, or husk, of the Nut Ivory (Fig. 39), presents a 

 very fine view of the arrangement of the sclerogen ; it is only neces- 



