LECT. VIII.] HOLLOW HERBACEOUS STEMS. 427 



and the boundary of the one compressed that of 

 the other; each mass exceeding in length the 

 space it is intended to occupy. In the old stem, 

 however, when the pulp which constitutes the pith 

 becomes dry and the cells empty, the diaphragm 

 is condensed to a thin opaque plate. 



In both kinds of hollow stems, the sap vessels 

 are spirals, formed of one or two flat, entire or 

 punctured threads ; while the proper vessels, as far 

 as I have been able to observe them, are simple, 

 transparent, cribriform tubes. The number of 

 vessels varies in each fasciculus, generally in- 

 creasing with the growth of the plant, and conse- 

 quently the same fasciculus consists of a differ- 

 ent number of vessels, according as the part in 

 which it is situated is nearer to, or farther from 

 the root. Thus, in a mature stem of the Gourd, 

 M. Kieser, who took the trouble of counting the 

 sap vessels, says the number in each fasciculus, 

 near the top of the stem, seldom exceeds six or 

 seven, but below the second knot it is nineteen, 

 and still greater below the third. In the centre of 

 the stem the number of vessels in each fasciculus 

 is twenty-three, and near to the root twenty-nine, 

 if the stem be examined in autumn. But besides 

 this diversity with respect to number, Kieser as- 

 serts that the size and general characters of the 

 vessels vary. At an early period, or near the sum- 

 mit of the plant, he says they are simple spirals ; 

 lower down they are of a larger size and annular 



