1885.] NEW-YORK MICROSCOPICAL SOCIETY. 87 



is very interesting, and I hope to make it plain by some models. 

 The statement in the text-books, that, when the pits are forming, 

 they have at first a septum which in a short time breaks away 

 and leaves a free communication, needs modification. Here are 

 two blocks, models, marked D and E, representing, on a scale 

 of four thousand diameters, tangential sections respectively of 

 the thin-walled and the thick-walled trachei'des. In the latter, 

 especially those which are nearly closed, the septum, generally 

 thickened centrally, will be found intact, as shown by block E. 

 In the thin walls, the septum will be found lying against the 

 dome, and is sometimes very difficult to distinguish ; and, often, 

 it is removed in cutting the specimen. 



The cells of the medullary system are uniseriate, — except 

 those which enclose a resin canal, — and are of two classes, mar- 

 ginal and central. The former have, by some, been called tra- 

 cheides ; but in American species they do not merit that classifi- 

 cation. Small lenticular cavities with septum are visible. The 

 communications between these cells and the upright trachei'des 

 are delicate and interesting. For we see round portions of the 

 wall projecting inward, forming little domes with central open- 

 ing and projecting orifice ; then, under each dome, a septum ; 

 then funnel-shaped openings extending to the lumen. All these 

 appear in transverse section as delicate, lenticular cavities. 



The central cells of the medullary system are quite different. 

 Their walls are more or less irregular, their ends curved, their 

 openings large ; and in transverse section these walls apparently 

 project into the lumen of the upright trachei'des, the limiting 

 lamella only not being perforated. This is not the case with all 

 the conifers. 



Our tracheide model illustrates yet other points. You see 

 these transverse indentations, and these openings. The former 

 show the position of the central rays of the medullary tract. 

 Sometimes there are only two rays, sometimes four, eight, or 

 nine : no regularity in the number has been found. The open- 

 ings show the form of the communications into the lumen. The 

 small openings at each side of the large central openings, belong 

 to the marginal rays of the medullary tract. The ends of the 

 tracheide are rounded, pointed, and, when directly in contact 

 with the medullary tract, sometimes straight. 



In the tangential section, the sides of the tracheide are some- 



