34 BIOLOGICAL LECTURES. 



internal structures, which are often most beautifully preserved. 

 The main features are as follows : 



The stem exhibits a concentric structure comparable with 

 that which is shown by the larger Laminariae, a feature which 

 in the first instance' led to a misinterpretation of their true 

 nature, as it appeared to indicate an exogenous stem. This 

 view was still further strengthened by the appearance of cer- 

 tain radial canals simulating medullary rays, but of very irregu- 

 lar width and indeterminate length, and wholly lacking definite, 

 limiting walls. Such spaces reappear in N. Ortoni, N. Crassum, 

 and others, where they take the form of nearly isodiametric 

 openings in the body of the structure, through which they are 

 scattered without any definite order or apparent connection 

 with the larger elements of the organism. They nevertheless 

 appear to be intimately connected with the smaller and second- 

 ary elements, as will be shown presently, and it is therefore 

 quite possible that, as suggested by Barber, they were designed 

 to perform a certain role in the internal aeration of the plant. 



The principal structure consists of tubular, non-septate cells 

 having a diameter upv/ards of 67 /i, but of indeterminate 

 length and loosely interlaced, so that their general direction 

 coincides with the axis of growth. These large cells of the 

 medulla branch into smaller hyphae which form an intercellular 

 plexus. The hyphae are chiefly from 4-6 ^ in diameter, though 

 sometimes much less. They appear, in some cases at least 

 (N. Storriei), to be septate, and their derivation from the large 

 cells of the medulla appears to occur chiefly in the immediate 

 region of the medullary spaces which they occupy in the form 

 of a very loose web. The majority of the species exhibit 

 minor differences only, but in N. Ortoni we observe the occur- 

 rence of well-defined trumpet hyphae, which not only estab- 

 lish a certain degree of affinity with algae of the type of 

 Macrocystis, but also serve to suggest its possible generic 

 separation from the other known forms. 



Our interest centers in the fact that these plants have no 

 modern representatives in any way comparable with them in 

 point of size. Occurring as they did so early at the Upper 

 Silurian, their gigantic dimensions and somewhat highly spe- 



