268 PROF. W. C. WILLIAMSON ON THE 



sterns^ almost undistinguishable from those of the secondary 

 medullary rays (fig. 3_, g) . At the same time ordinary 

 Calamites vary extremely in the length of these primary 

 rays, and I have transverse sections in my cabinet which, 

 in this respect, approximate very closely to what I find in 

 ray sections of C alamo dendr on. 



Comparing the three forms of organization illustrated 

 bv figs. 4, 5 and 6 we find them nnmistakeably con- 

 structed upon a common plan, even as regards the most 

 important of the details. The differences between the 

 vascular or xylem elements of the three examples have no 

 more than specific value. The chief distinctions between 

 figures 4 and 6 are to be found in what I term the primary 

 medullary rays. What in the ordinary Calamites we 

 have seen to be entirely composed of parenchyma, in the 

 Calamodendron consists of prosenchymatous fibres largely 

 intermingled with radial parenchymatous laminse. My 

 numerous examples of very young and minute Calamites 

 show me that, in them, these primary medullary rays origi- 

 nate in exactly the same way as they do in the first year's 

 growth of any ordinary exogenous stem * ; whilst, as is 

 also the case in these Exogens, the peripheral ends of these 

 primary rays become undistinguishable f rom the secondary 

 medullary rays in the more external layers of older stems. 

 These identities justify my designating both medullary 

 rays. The only question of importance therefore to be 

 asked is. Does the alteration of their composition seen in 

 Calamodendron, compared with what we find in Calamites, 

 materially alter the character of these organs ? I con- 

 clude that it does not. In the first place, it is indisputable 

 that fig. 5, my so-called Calamopitus, is but a very slightly 



* De Bary applies to these oi-gans iu Phanerogams precisely the same terms 

 that I have for years applied to those ol the Calamites. See 'Comparative 

 Anatomy of the Phanerogams and Ferns,' English Translation, p. 235. 



