LEPIDUDENDREAE. 22$ 



furnished with prominent angular projections of varied shap'e, the transverse 

 sections of the leaf-cushions, and each projection receives a vascular bundle. 

 Dichotomies, such as have been described above, are frequent. In the pre- 

 parations before me I find the epidermis preserved, but in many cases 

 already partly separated by a fissure caused possibly by maceration. The 

 central bundle must be discussed at somewhat greater length. In all 

 the transverse sections of branches from Burntisland which Williamson was 

 able to examine, the centre was formed of parenchymatous tissue, the 

 periphery of a closed ring of tracheides which showed on the inside a slightly 

 irregular boundary-line. In the smallest branches the mass of central 

 parenchyma was small, and was surrounded only by two or by a few layers 

 of tracheides. In larger branches it was broader, the ring of tracheides 

 thicker, and consisting in the radial direction of from five to eight elements. 

 Lastly, still thicker branches showed the presence of secondary wood, and 

 the larger the transverse section of a stem or branch, the more voluminous 

 was the mass of parenchyma of the central strand and the broader the layer 

 of tracheides surrounding it. The same results were obtained from the 

 examination of the Arran material. Here in the very smallest branches 

 there was no parenchymatous centre at all, the entire central strand was 

 composed of one form of tracheides ; the medullary tube made its appear- 

 ance as the branches grew larger. I have before me a young branch of this 

 kind from Halifax for which I am indebted to Mr. Cash ; its structure agrees 

 perfectly with that of a specimen figured by Williamson *, and shows a 

 closed trachea! strand. If then all the remains from Burntisland or Arran 

 are brought together to form one vegetable species, as is done by William- 

 son, and if further the whole of the known branches and stems are combined 

 in sequence of time to make up a course of development which each of them 

 would have passed through if undisturbed in its vegetation, we are driven in 

 presence of the actual conditions to the conclusion that the central strand 

 possessed unlimited growth, which is manifested in the enlargement of the 

 inner parenchyma, and in the growth of the outer layer of tracheides, both in the 

 surface direction by intercalation and in thickness by increasing the number 

 of the elements in the radial direction. And therefore Williamson also has 

 concluded, and quite logically frcm his position, that in the Arran Lepido- 

 dendron, for example, the central strand at first solid begins by forming 

 parenchyma in its centre and then goes on increasing by growth, and that 

 this growth and the constant increase in volume which results from it is in 

 no way retarded or stopped even by the development of secondary tissue. It 

 is this latter point, as Renault 2 justly urges, which is not very intelligible, 

 since it is difficult to see how the secondary wood can allow space for the 

 further growth of the primary strand ; and we know that this growth in 



1 Williamson (1), \, t. 14, f. i. 2 Renault (1), p. 247. 



