LECT. VIII.] HOLLOW HERBACEOUS STEMS. 431 



although the ultimate changes may take place in 

 the cells ; an hypothesis, however, which I am by 

 no means prepared to admit. 



Pursuing our inquiries into the structure of 

 herbaceous roots, we find that the analogy be- 

 tween these organs and the stem is less close 

 than in ligneous plants. This arises in many in- 

 stances from the difference in the duration of the 

 life of these parts in herbaceous plants ; for the 

 root may be biennial or perennial, when the stem 

 or stems are annual only, or dying in the autumn 

 and giving place to others, which rise from the 

 same root in the succeeding spring. 



When the root is annual only, as, for ex- 

 ample, that of the Sweet Pea, Lathyrus odoratus, 

 it consists of a cuticle enclosing a thick cellular 

 cortex, apparently devoid of vessels; and a cen- 

 tral part, which is composed of a bundle of sap 

 vessels, or rather of several bundles, arranged so 

 as to form an irregular six-rayed star, embedded 

 in a mass of small oblong cells, through which 

 six fasciculi of proper vessels descend, almost 

 touching the cortex, and situated in the spaces 

 formed by the rays of the central fasciculus of 

 sap vessels. The sap vessels are all punctuated 

 spirals * ; but those in the centre are smaller 



* This is the only form of sap vessel I have observed in an- 

 nual roots ; and as Kieser, also, found punctuated spirals only 



