SPECIAL MORPHOLOGY : RHIZOCARPE^E. 



205 



gradual evolution into a plant. Finally, in the third place, I had seen, 

 in a transverse section of a seed-bud of Salvinia, which had lain some 

 time to germinate in water, a filiform cell which ran obliquely through 

 the green cellular tissue from a somewhat lateral point of the embryo-sac, 

 and hung out considerably beyond the seed-bud, but appeared to be torn 

 away here. As soon as I had an opportunity, I made a minute investi- 

 gation, and soon had the satisfaction of discovering the whole process of 

 germination, such as I have described it in the paragraph, first in Sal- 

 vinia, and afterwards, with less trouble, of confirming it, in Pilularia. 

 In Salvmia, with the exertion of all my patience, I have only succeeded 

 three times in making the section so fortunately as to lay open the entire 

 course of the pollen -tube. Since it runs obliquely, and the minute seed- 

 buds present externally no points by which they can be held, the section 

 becomes naturally a matter of chance. In the somewhat later stage of 

 development of the seed-bud the form of the nuclear papilla becomes a 

 sufficient guide for an accurate section. In Pilularia (fig. 149.), on the 



149 



contrary, I have often succeeded in extracting, in a free condition, the 

 pollen -grains (fig. 149. B} with the vesicular expanded extremity of their 

 tube (E) in the seed-bud (C) perfect and uninjured. Here it is not 

 very difficult to trace the whole course of development. Three or four 

 pollen-tubes usually penetrate into one seed-bud here, but only one 

 passes deeply down and becomes the embryo (fig. 149. B, b, C, d, E) : 



149 Pilularia globuUfera. A, Transverse section through a seed-bud at the commence- 

 ment of development : a, gelatinous coat ; b, coriaceous coat ; c, embryo-sac, filled 

 with starch and oil-globules ; d, nuclear papilla. B, Pollen grains : cr, fresh from the 

 pollen-sac ; b, swollen in water, and beginning to produce tubes. C, Upper part of the 

 seed-bud, after the penetration of the pollen-tube (d) : a, coriaceous coat ; b, embryo- 

 sac ; c, nucleus and nuclear papilla ; k, layer of cells which separates the pollen-tube 

 from the embryo-sac. E, Pollen-tube prepared free, from C : at the upper part it 

 exhibits the now uncovered portion which was enclosed in the outer pollen-membrane ; 

 in the middle, the slender, proper tube ; and below, the broadly expanded portion, now 

 filled with cellular tissue, which developes into the embryo. D, Upper end of the 

 seed-bud, in a still more advanced condition : a, coriaceous coat ; b, embryo -sac ; c, nu- 

 cleus and nuclear papilla expanded into a sac, through the development of the embryo ; 

 rf, stem-end of the embryo (e) ; g, first leaf (cotyledon) ; h, pollen-tube ; /, first axil- 

 lary bud ; filiform elongated external cell of the nucleus ; k, layer of cells, which 

 separates the embryo from the embryo-sac. 



