34 



VEGETABLE PHYSIOLOGY. 



[LESSON 11. 



part of one duct, and the upper portion of another is shown together 

 with two perfect cells : the pores are bordered pores. 



199. In the apple tree another form of porous vessel is found ; 

 a porous vessel with very minute pores placed in the interstices of a 

 double spiral, suddenly bulges out into an inordinately large cell, 

 destitute of pores, and having a number of lines, or bars, running 

 irregularly across it (Fig. 62) ; it does not appear that there is any 

 membrane between these bars, or lines, to connect them. 

 FIG. 61. 



FIG. 62. 



FIG. 63. 



Porous duct, Locust Porous vessel, Apple. Porous duct, Basswood. 



200. But the most beautiful ducts or vessels of this kind, are 

 found in the Basswood; here, they exist in great number and of 

 considerable length, having a uniform diameter for the greater part 

 of their course ; the ends are pointed. 



201. They consist of a singularly beautiful double-spiral thread 

 (Fig. 63), in the meshes of which one, and sometimes two ducts are seen. 

 The whole of this structure is confined in a tube of cellulose, which, 

 under the microscope, presents a very glistening appearance. 



LESSON XI. 



SILICA. 



202. This mineral, so abundantly found in all kinds of soils, uni- 

 versally throughout the surface of the Globe, is of the greatest conse- 

 quence to the vegetable kingdom. 



203. We have already seen that certain vegetables have a para- 

 mount necessity for the Carbonate of lime ; with its assistance, as an 

 article of food, they nourish, and luxuriate without it, they perish. 



204. The carbonate of lime rarely exists in its pure, original state, 



