164 BOARD OF AGRICULTURE. 



been adduced concerniDg the circulation of sap, and the mode 

 of vegetable growth. 



The simplest and minutest of plants consist of single hol- 

 low globules of vegetable membrane, in which no pores nor 

 openings of any sort can be detected by the most powerful 

 microscope. These globules are often so minute as to be in- 

 dividually undistinguishable by the unaided eye, and yet are 

 so numerous as to give their color to large expanses of snow, 

 on which they flourish-, or to water which is inhabited by 

 them. These globules are called vegetable cells, and such 

 plants are unicellular. In most species, however, the mature 

 individual, while always originating from a single embryonic 

 cell, consists' of innumerable cells ao-crreo-ated into some char- 

 acteristic form. Thus, among the fungi we have species of 

 mould which are unicellular, and single pufi'-balls containing 

 thousands of millions of cells. 



When vegetable cells are thus crowded together, they be- 

 come variously modified in form, and Avhile in the lower or- 

 ders they remain entirely closed and disconnected, except by 

 the cohesion of their imperforate membranes, in the higher 

 orders, as in our common trees, they are often converted 

 into continuous tubes by the absorption of the intervening par- 

 titions. These tubes are always formed parallel with the axis 

 of growth, and, along with woody fibre, constitute what may 

 be styled the warp of all timber. The woof, running from 

 the centre to the circumference of every woody stem, con- 

 tains only flattened, imperforate cells. These are always 

 present and often very abundant, and constitute what is 

 known to mechanics as the silver grain of the wood of maple, 

 beech, and other so-called exogenous plants, to wdiich, for 

 the sake of brevity and clearness, this discussion will be 

 confined. 



AVhen ordinary cells are united into a more or less 'ex- 

 tended structure, they constitute what is known as cellular 

 tissue. This may be soft and pulpy, as in the flesh of an 

 api^ile, loose and tender, as in the pith of an elder, firm and 

 tough, as in birch-bark, or hard and brittle, as in the shell of 

 a hickory-nut or the stone of a peach. 



The tissue of the inner bark,' consisting of very long and 

 narrow cells overlapping each other at the ends, is called 



