1892.] NEW-YORK MICROSCOPICAL SOCIETY. 81 



bundles for a series of years. During the first and second years 

 the libriform tissue composes the bulk of the fundamental mass ; 

 in the succeeding annual zones it diminishes and may nearly 

 disappear, being replaced by tracheids and vessels. The spiral 

 tracheids figured at T are perhaps the most abundant tissue in 

 the plant. Retaining their original cambium form, they illustrate 

 especially well the transition from tracheid to vessel. If they 

 exist as vessels their irregular course indicates at once where the 

 septa were, as at X. As noted above, they differ from the spiral 

 tracheids of the medullary sheath in their large and thickened 

 walls, while the spirals often branch and the vessels become pit- 

 ted (Fig. 9). No measurements will be given when the indi- 

 vidual element is figured, the illustrations being designed to 

 represent the average size of the cells. The pitted vessels, a 

 small one of which is figured at V, are the most noticeable of the 

 individual elements, being easily seen with the eye, and usually 

 forming, after the sixth year, the bulk of the wood. They have 

 much the same arrangement as in the Quercus rtibra, the concen- 

 tric circles seemingly confined to the autumn wood and fig- 

 uring sparingly in the denser portions. The size of the d-ucts 

 varies; comparatively small in the first zone, th«y increase annu- 

 ally and ifeach their maximurn growth in from eight to ten years, 

 often having a diameter of from 300 to 400 /^. The walls are 

 much pitied with discoid markings. Fig. 10 represents a vessel 

 separated by Schultze's fluid and viewed from without. The halos 

 seem to be oval, arranged in rows, and alternating in the rows 

 with one another. The canals are narrow slits, usually extending^ 

 quite across the disc, and are partially concealed by the thicken- 

 ing membrane of the vessel. Tyloses is the rule with these ves- 

 sels. The walls of the intruded parenchyma often become much 

 thickened and pitted. The frequency is probably due to the fact 

 that parenchyma usually borders these vessels, and the numerous 

 slits afford it an easy ingress. These large and numerous tubes 

 serve important mechanical ends in giving lightness and strength 

 to the iplant. A rough measurement of a cross-section places 

 the area of the tubes as one-third that of the plant. 



The wood parenchyma shows at P. In the variation of its di- 

 mensions and form it well exemplifies its name — parenkeo. The 

 cells are irregularly pitted when contigufous to vessels or medul- 



