94 STRUCTURE OP EXOGENOUS STEMS. 



place to the woody portion : and whilst at first it has to impart 

 nourishment to the organs which are growing from it, this 

 function is performed more perfectly by the vessels, as soon as 

 they are developed in the stem, and the pith becomes of no fur- 

 ther use. The pith of a branch is always an extension of that 

 of the branch or stem from which it sprang ; and if the latter be 

 cut through, just where a bud is rising from it, this will be seen 

 at first to consist, almost entirely, of a kind of prolongation of 

 the cellular portion, which occupies its centre. The pith of trees 

 is applied to no important use. One curious product is obtained 

 however from the large pith, which constitutes nearly the entire 

 stem of an herbaceous plant ; this is the substance known as Rice 

 Paper, which is made by cutting the soft portion of the stem 

 vrith a sharp knife, in a spiral manner, so as to cause it to spread 

 out, as if a sheet of paper were being unrolled from a round 

 ruler. It is then flattened out by pressure ; but its character, 

 as cellular tissue, like that of other piths, may be easily shown 

 by magnifying a small portion of it. 



128. The pith is surrounded by the woody layers, the num- 

 ber and thickness of which depend upon the age of the branch 

 or stem. In the herbaceous stem, the woody portion (as already 



stated) presents itself in the form of 

 distinct strings, arranged in a circle 

 between the large pith and the ex- 

 ternal skin or bark. Each string is 

 separated from its neighbours by a 

 prolongation of the pith, which thus 

 maintains its connection with the 

 bark. In the stem of a second 

 year, however, these bundles are 

 found to form a second ring, en- 



FIG. 41. DIAGRAM IILUSTRATING THK c l os i n g the first, but Still beneath 



FORMATION OF THK STKM, , pith; 6, . . 



bark ; c, c, c, plates of cellular tissue the bark ; which is carried out- 



posed between these. prolongations of the pith still exist ; 



but they become narrower in each ring, and at last appear merely 



