LKCT. VII I.] STRUCTURE OF HERBACEOUS STEMS. 421 



renchyma e., . and which is altogether composed 

 of hexagonal cells, that become larger and more 

 regular as they approach the centre; and supply 

 the place of the pith. The green secretion in the 

 pulp of the woody layer slightly tinges the fluid 

 in the cells of the pith to some distance from the 

 circumference ; but, the colour being more and 

 more diluted, the central cells are completely co- 

 lourless. The sap vessels are spiral, each being 

 composed of three distinct filaments. The proper 

 or returning vessels are cribriform and extremely 

 delicate. 



As in the former division of this description of 

 herbaceous stems, there are many modifications of 

 structure, so, in some instances, in this division, it 

 approaches more closely to, and in others recedes 

 more widely from, the characters of that of trees. 

 Thus, in the stem of Endive, Chicorium Endivia, 

 which Malpighi has described *, the vessels near 

 the circumference are disposed in lines directed 

 towards the centre, and are separated by small 

 septa of compressed cellular substance; but the 

 interior, which are also the larger, extend in 

 rays a considerable way through the pith. In the 

 stem of Lettuce, Lactuca sativa, which may be 

 regarded as the intermediate of the two divisions 

 already noticed, the sap vessels being in distinct 

 fasciculi like those of the first, but arranged in 



* Anatomia Plantarum, p. 25. 

 EE 3 



