206 



MORPHOLOGY. 



150 



from the shortness of the pollen-tube the granules themselves are situ- 

 ated quite near, upon the seed-bud ; by degrees they lose their external 

 membrane, and then appear, like three or four pyrifbrm utricles, issuing 

 from the seed-bud (tig. 149. D, h), which Mtiller* actually imagined 

 them to be. The course of development of Marsilea I have not yet had 

 the opportunity of making out; what Esprit Fabref has published on 

 the subject I unfortunately only know from Meyen's Year Report J, 

 where the account, whether from the fault of the author or the reporter 

 I know not, is very superficial and imperfect. The great agreement of 

 the structure with Pilularia leads to the expectation that no essential 

 deviation will be found to exist. There are two more points connected 

 with the course of development to which I must call attention. The 

 pollen-tube, as has been stated, does not come 

 into immediate contact with the embryo-sac, since 

 the apex of the latter is closely invested by a 

 simple layer of green cells (fig. 149, C, k, D,k). 

 Before the nuclear papilla is fully formed, the 

 membrane of the embryo-sac is very tough, 

 and almost coriaceous ; subsequently it expands, 

 so far as it is covered by that cellular layer 

 of the nuclear papilla, hemispherically (in Sal- 

 vinia\ or even into a long cylinder rounded off 

 above (in Pilularia), and thus exhibits at this 

 region an extremely delicate membrane, which is 

 continuous below with the unaltered tough one 

 (fig. 149. Z>, b). The pollen-tube, which pene- 

 trated and has become vesicularly expanded, 

 forms a very delicate investment over the deve- 

 loping embryo for a long time after (fig. 149. e\ 

 which even remains attached up to a very late 

 period on the point where the pollen-tube entered, 

 which can always be recognised by the three to 

 five contiguous cells appearing brownish as if 

 dead. Two extremities may be distinguished in 

 the part of the pollen-tube which has entered : the 

 upper closed end, which went first in the act of pe- 

 netration (fig. 150. //); and the other, which loses 

 itself externally in the pollen-granule (fig. 150. x). 

 The former is firmly applied upon the layer of 

 cells investing the embryo-sac ; it may be called 

 the stem end, and the other the root end. In the rest of its periphery 

 the pollen-tube, and therefore the embryo, remains quite free. Close 

 beside the stem end, immediately at the point where its connection with 

 the cellular layer of the nuclear papilla ceases, is now developed the bud 

 (figs. 149. A /, 150. &), which may here be regarded as a first lateral 

 bud, an axillary bud 9f the first leaf (figs. 149. D, g, 150. rf), or coty- 



* On the Germination of Pilularia globulifera, in the Flora, 1 840, No. xxxv. p. 545. 

 (Otherwise an excellent treatise, with many very accurate observations.) 

 t Ann. des Sc. Nat. 1837, April, p. 221. 

 j: Wicgmann's Archiv, 1838, vol. ii. p. 82. 



150 Pilularia globulifera. A considerably advanced stage of development, a, Seed- 

 bud ; 6, axillary bud of the embryo ; c, the nucleus, expanded into a sheath for the 

 embryo; d, first leaf; e, first root; .r, pollen-tube; y, collar-like thickening of the 

 coriaceous coat. 



