THE STEM OF VASCULAR PLANTS. 199 



interval, either leaves, branches, or roots : on the con- 

 trary, in the knots, the central cavity occupied by the 

 cellular tissue is interrupted, the fibres cross in the 

 horizontal direction ; they give rise to a sheathing leaf, 

 and in the axil of this is always found a bud, which is 

 developed or not according to circumstances. It is from 

 this knot also that all the adventitious roots spring in 

 grasses, when their stems or lower branches are either 

 prostrate or subterranean — as, for example, in Tritieuvi 

 repens and Panicum Dactylon, where the lower branches 

 are prolonged horizontally undergrovmd, and commonly 

 bear the name of roots. Frequently, also, in grasses 

 with upright stems, the lower knots are so near the 

 ground that it often happens that they shoot forth roots. 



The distance between the knots is very variable, not 

 only in different grasses compared together, but also in 

 individuals of the same species, and in the knots of the 

 same individuals : in general they are more distant when 

 the stems grow in a fertile soil, and upon the same stem 

 they are seen nearer together at the base, and more 

 separated above. We also remai'k, that the closer the 

 knots are, the more easily is the axillary bud of their 

 leaf developed ; this takes place in the grasses which 

 ramify principally at the base of the stem. 



When the lower internodes are very short, it often 

 happens that they swell up so as to form a kind of dila- 

 tation, which, covered over by the sheath of the leaf 

 which it has distended, resembles the bulbs of the Lili- 

 acea9 : this has caused some of the Graminese to be called 

 bulbous ; such is, for example, Hordeum strictum, which 

 is often bulbous, and then it receives the name of 

 H. bulbosum; Phleuni nodosum probably only differs 

 from P. pratense by this same phenomenon taking place. 



It sometimes happens that the swollen knots are sepa- 

 rated by a short internode, and then the series of these 



