294 ME. D. T. GWYNNE-VAUGHAN ON THE 



So that, although there is no separation of plerome and periblem in the apical cone itself, 

 a little lower down in the young stem such a distinction becomes evident. 



Insertion of the Adventitious Moots. 



In the several genera examined by me, and probably throughout the w^hole order, the 

 rhizomes are beset with adventitious roots springing- in clusters from the bases of the 

 petioles. In NeliimUiim they are present at the base of every leaf, so also, the first or 

 acicular leaf excepted, in VicfoiHa, Nymphma, and Nupha7\ The number of roots 

 belonging to each cluster varies considerably, being greatest (15-20) in the mature 

 rhizome of Victoria (PL XXI. fig. 10); on the other hand, in Nymphcea and Nuphar 

 each group contains from 3 to 7 only. 



These roots are given off from certain vascular b^lndles especially set aside for that 

 sole purpose, which are separated off from the central vascular mass, and run apart 

 from it in an upward direction in the outer region of the rhizome into the dorsally 

 protuberant bases of the petioles, W'here they lie on the outer side of the leaf-traces 

 (fig. 11). 



That certain bundles were thus set aside for the express purpose of bearing the 

 adventitious roots was observed by Trecul in Nuphctr luteum in 1845 *, in Victoria 

 regia by Henfrey in 1852 f, and in JSeli(^mbium speciosum by Wigand in 1871 J. I 

 have further discovered that the different species show interesting variations in the 

 manner in w^hich these bundles are arranged. Eor instance, in Victoria regia (fig. 12) 

 all the root-bearing bundles belonging to the same leaf-base are grouped togetlier so as 

 to form a structure having the appearance of a definite and distinct stele, consisting of 

 a number of vascular bundles, usually about 20, arranged in a complete ring, with the 

 phloem groups well marked and generally distinct from each other, while the xylem 

 elements form a more or less continuous ring wdthin them. Other xylem elements are 

 scattered throughout the central parenchyma of the stele, so that no definite medulla is 

 distinguishable (fig. 13). The xylem is centrifugally developed, although, since it is 

 entirely composed of long pointed tracheides, longitudinal sections are necessary to 

 settle this point. The whole stele is surrounded by a clearly marked endodermis. This 

 stele bears the adventitious roots in acropetal succession on its outer side, while it itself 

 terminates in the youngest root or roots. 



Owing to the delay in the further development of the adventitious roots after their 

 first formation they possess pedicels of a considerable length, and do not obtain individual 

 cortices until they have reached the extreme periphery of the leaf-base, where the point 

 at which they do so is marked in each root by the presence of a hemispherical transverse 

 diaphragm formed by a layer of small closely packed cells. These diaphragms enable 

 the roots to be broken off the stem, leaving behind a clean-cut scar and not a tear. In 

 appearance the pedicels resemble the structure of a stem stele much more than that of a 

 root (fig. 14). They have a ring of several indistinct groups of phloem, and wdthin this 

 an almost continuous ring of xylem elements surrounding a small medulla ; the whole 



* Ann. des Sc. Xat., se'r. 3, tome i. p. 293 et seq. t Ptil. Trans, 1S52, p. 289 et seq. 



X Botaniscbe Zeitung, No, 48, p. 821. 



