514 HEY. GEORGE HEKSLOW ON A THEORETICAL 



From this zone arises the adventitious roots ; while he figures 

 the attachment of the roots to this zone in Nupliar luteum as a 

 funnel-shaped process consisting of branching tracheides, not at 



all unlike what takes place in endogens ; indeed, M. Trecul 

 observes with regard to the adventitious roots : — " La structure 

 et l'accroissement sont aussi ceux des racines des Monocotyle- 

 dones." . . . "Au point d'origine de ces racines, les faisceaux 

 du centre sont unis entre eux par un cercle vasculaire, qui relie 

 le s) T steme des vaisseaux de toutes les racines adventives qui 

 naissent du memeplan transversal " 



Other Exogenous Stems with scattered Cords. — The Nymplice- 

 accce, however, are by no means the only exogenous group with 

 scattered medullary bundles. There are doubtless other causes 

 besides water which may bring about a disarrangement of the 

 cords ; one of these appears to be a climbing habit. The relaxation 

 of the effort required for self-support results in an alteration of 

 the characters of the xylem zone, in that the wood fibres become 

 less lignified and the vessels larger and more numerous. Accom- 



* 



panied with these features is a different arrangement and often 



origin of fibro-vaseular cords. The Cucurbitacece and JPiperacete, 

 for example, both illustrate the occurrence of medullary bundles ; 

 while that of Peperomia is particularly interesting from the 

 present point of view, as it is practically identical with that of 

 Commelina f. 



We have already seen some analogous degradations between 

 the embryos of parasites and of aquatic plants ; the former also 

 furnish others in the structure of the stem. Thus species of 

 Viscum have an irregular circle of very unequal-sized cords. 

 Myzodendron has two concentric circles of completely isolated 

 ones. Lastly, Cynomorium coccineum and Helosis guyanensis 

 have them scattered through the ground-tissue exactly like any 

 endogenous stem J. 



There are, of course, many other instances of medullary cords 

 in exogens, both m the stems and petioles, as in several of the 



Ann. Sci. Nat., Bot. 4 ser. i. p. 1C>3, pi. 14. fig. 13, " Eecherches sur la 

 structure et le developpement du Nitphar lutea ? and Ann. des Sci. Nat. 3 str. 

 torn. iv. 1845, p. 280, pi. 10. fig. 4, pi. 11. fig. 11. 



t See descriptions and figures in De Bary's Comp. An. of Phan. and Ferns, 

 pp. 248 and 2G9. 



X Fur figures, see Chatin, Anat. Comp. des Veg. vol. ii. pis. xciii. and icvii. 



