784 TRANSACTIONS OF SECTION K. 



xylem of the lower portions of these strands is lignified the lignification seldom 

 extends beyond the base of the integument, but sometimes one or more spirally 

 thickened tracheitis are found running up into it. The tip of the inner integu- 

 ment forms the micropylar tube, traversed by a slit-like micropyle ; there is an 

 external ridge-like outgrowth from the tube which turns back and fits tightly 

 over the top of the middle covering. 



The embryo sac is developed between the levels of origin of the two inner 

 coverings and grows as the intervening region stretches. It projects into the free 

 portion of the nucellus. A well-developed pollen chamber is present in the young 

 o\ ale. As it decays the nucellar tissue below it becomes lignified and forms a 

 hard beak. 



The radial structure of the seed, the presence of a pollen chamber, and the 

 small development of the free portion of the nucellus are all points of striking 

 contrast to Welwitschia, and probably point to the more primitive character of 

 the Gnetum ovule. The two integuments in both seeds are compared with the 

 two integuments of Lag eno stoma, the vascular system of the inner integument 

 having suffered considerable reduction. 



6. On the Diversity of Structures termed Pollen-Chambers. 

 By Professor F. W. Oliver, F.R.S. 



The object of this paper was to show that in several of the lesser-known 

 fossil-seeds of Coal Measure times {Conostoma oblongum , C. anglo-germanicum , 

 and perhaps Gnetopsis), belonging in all probability to the Lyginodendreae 

 section of Pteridosperms, the structure of the nucellar apex was much more 

 complex than in either Lagenostoma or Physostomo. This elaboration arose 

 through the excavation of the plinth below the ordinary pollen-chamber, so 

 that a second chamber was provided into which the pollen was received and 

 in which it doubtless matured the male gametes. The upper chamber, evidently 

 homologous with the pollen-chamber of Lagenostoma, is thus vestigial in char- 

 acter. The pollen-chambers of Trigonocarpvs and other seeds attributed to 

 the Medullosae section of Pteridosperms, and also those of Ginkgo and recent 

 Cycads, were considered from this point of view, and the suggestion was made 

 that perhaps the nucellar beak might also be a vestigial primary pollen-chamber 

 which had become functionally replaced by a deeper-seated cavity, in which the 

 pollen grains pursued their further development. 



7. On the Stock of Isoetes. By Professor William H. Lang, D.Sc. 



A re-examination of the anatomy of the stock of Isoetes has confirmed the 

 view of Von Mohl that the roots are borne in regular order on a downwardly 

 growing region. The root-bearing region of the stele is not due to secondary 

 modification of the base of the leaf-bearing portion, but is, from the first, a 

 distinct region. 



The apex of the stem is well known to be seated in a deep depression, and 

 the so-called secondary growth of the cortex is required to carry out the leaves 

 from the central region and thus leave room for new leaves to arise. The 

 basal root-bearing region of the plant corresponds to such a depressed apex 

 with the root-bearing surfaces facing one another and congenitally united. 

 The grooves separating the lobes of the stock come apart by the separation of 

 these united surfaces, and this continued process of splitting brings the young 

 roots to a free surface. 



While greatly modified by the slow and stunted growth of both the leaf- 

 bearing and root-bearing regions of the stock, the whole axis of the Isoetes 

 plant can be compared with that of Lepidodendron or Pleuromeia. The root- 

 bearing region appears to be strictly comparable with the Stigmarian base of 

 these plants. Such a comparison, though neglected of late years, was made by 

 Williamson in his monograph on Stigmaria. 



