52 EXPLANATION OF PLATES. 



Fig. 7. End of a young leaf, seen in optical longitudinal section, X 245. a, apical coll; ep, young epidermis; 



me, mesopliyll ; fi, flbro-vascnlar bundle of the rachis. 

 " 8. Transverse section of the complete bundle of the secondary rachis, X 125. sh, l)undle-sheath. 



" 9. Transverse section of the end of the bundle of the base of the petiole, X 125. 



Figs. 10-14. Successive stages in the development of the stomata, seen from above, X 500. 2, preliminary cell ; 



y, mother-cell of the stoma. 

 Fig. 15. Apex of the stem of a mature plant, seen from above, X 150. a, apical cell; the six youngest segments 

 are numbered, beginning with the youngest. 



PLATE 7. 



Fig. 1. Longitudinal section through the apex of the stem of a mature plant, X 150. a, apical cell ; x, derniato- 



gen ; y, periblem ; z, pleromc ; in, medullary parenchyma. 

 " 2. Longitudinal section through the apex of a young plant with three fully developed leaves, X 245. Let- 



ters as in 1. 

 " 3. Surface view of the apex of a stem of about the same age, X 245. 



" 4. Part of a sieve-tube from the stem bundle, X 500. 



»< 5. A vertical section throngh the wall of a sieve-tube, X 500. 



" 6. Vertical section through the apex of a strong root of a mature plant, X 245. a, apical cell; c, root- 



cap; X, dermatogen; y, periblem; z, pleromc. 

 " 7. End view of the region of the apical cell of a medium sized root, X 245. The eight youngest seg- 



ments are numbered. 

 " 8. Transverse section of the younger part of a root, X 125. The original sextant walls are indicated by 



heavier lines. 

 Transverse action of the complete flbrovascnlar bundle of the root, X 125. 

 Cross-section of the pinna of a fertile frond, showing the revolute edges and the position of the sori, X 



30. 

 Vertical section through a very young sorus, X 150. 



" " " " somewhat older one showing the indusium (ui), X 60. 



Mother cell of a sporangium {in), X 250. 



Successive stages in the development of the sporangium, X 250. All in optical longitudinal section ex- 

 cept 19, which is seen from above. In all cases but 19 and 22 the plane of the annulus Can) coincides 

 with that of the paper. 

 Fig. 24. Young sporangium about the time of the absorption of the tapetum, X 200. Optical section. The re- 

 mains of the tapetal cells can be seen, and the mass of spore-mother-cells, sp. 

 " 25. Earlier stage in the division of the spores, X 375. 



" 26. A full-grown sporangium, X 125. an, annulus; .si, stomium. 



" 27. Ripe spores, X 2.50; a from above; 6, from the side. 



" • 28. Two lobes from the pinna of a dead fertile frond, showing the position of the sori, X 10. 



