ROOTLET BUNDLE. 29 



difficult to believe that a large root several inches in diameter does not possess 

 more rootlets than a small one whose diameter does not exceed a minute fraction of 

 an inch. Yet series of tangential sections, made from successive planes of a vascular 

 cylinder, display exactly the same number of such bundles, whether such sections are 

 made close to the medulla, or in immediate contact with the cortex. The primary 

 medullary rays through which these rootlet bundles emerge differ in this respect 

 from the secondary ones. The former do not increase in number with age. The 

 latter do so indefinitely. The two modes of growth, viz. individual dilatation and 

 meristemic multiplication of their elementary tissues, have caused each individual 

 rootlet to increase in size ; the same actions, taking place simultaneously in the 

 bark upon which the rootlets were planted, pushed those rootlets further and 

 further apart from one another, as is demonstrated by Plate III, figs. 6, a, and 6, b. 



Thus, some rootlets in my cabinet have only a diameter of '025, others reach 

 *4, the latter being sixteen times larger than the former, without any change what- 

 ever being made in their characteristic organisation. We shall see, what was first 

 demonstrated in my 'Memoir,' Part XI, p. 291 — 93 ('Phil. Trans.'), that this increase 

 in the diameter of each rootlet is accompanied by an approximately corresponding 

 increase of the number and diameters of the vessels forming the rootlet bundle. 



Plate XI, figs. 54 to 61,/, represent transverse sections of vascular bundles 

 from within the interiors of rootlets of different ages, each with more or less of its 

 investing cellular cylinder,/, and all equally enlarged 100 diameters. The differ- 

 ences between the magnitudes, both of the bundles and of their investing sheaths 

 are seen when we contrast figs. 5-1 — 6 with 58. In fig. 54,/, the formation of the 

 bundle begins with three extremely minute vessels or Tracheids (/'") closely com- 

 bined at one point of the wall of the investing cellular sheath,/' ; to which vessels a 

 fourth larger one,/, has been added centripetally. In fig. 55 a similar arrange- 

 ment exists, only we have here a fifth and yet larger vessel, /", added to the inner 

 side of /of fig. 54. In fig. 56 we have five Tracheids, between which number and 

 what we see in fig. 57 any number of examples could have been figured. Thus 

 far the absolutely monarch character of the vascular bundle is clear. 1 In fig. 57 

 the equally monarch bundle, /, is greatly increased in size contrasted with figs. 

 54 — 6. It now consists of at least eleven vessels, the smallest of which,/', retain 

 their monarch character as definitely as their representatives, /", in fig. 54. The 

 large size of the newer vessels,/, is conspicuous, and a similar enlargement is seen 

 in the cells of the sheath, /". In fig. 59 the number of the Tracheids has 

 increased to fifteen, and in fig. 58 to sixteen. All the last three figures show the 

 bundle to be as independent of the surrounding cellular sheath, except at the point 



1 This point is interesting viewed relatively to an opinion entertained by M. van Tieghem 

 respecting the origin of similar bundles amongst recent Lycopods, to which attention will again be 

 directed. 



5 



