STRUCTURE.] OVULAR SACS. 395 



the ovule results from the action of the pollen. For there 

 are many instances of considerable irregularity in the degree 

 of development of the ovule at the period of expansion of the 

 flower. From M. Decaisne's description of the ovule of 

 Viscum album, which appears to agree tolerably well with 

 that of an Himalayan species, it is, I think, evident that in 

 the earlier stages of its development it may defy observation, 

 since at one period it would seem to consist of nothing but a 

 single cell, scarcely, if at all, distinguishable from the cells 

 composing the surrounding cellular tissue." 



When the ovules are attached to the placenta by a kind of 

 cord, that cord is called the fumculus (Plate V. fig. 26. a), 

 and is a prolongation of the placenta. 



In the beginning the ovule is a pulpy excrescence (Plate V. 

 fig. 16.), appearing to be perfectly homogeneous, with no 

 trace of perforation or of envelopes. But, as it advances in 

 growth, it is gradually (Plate V. fig. 17. to 21.) enclosed in 

 one or two sacs or integuments, which are open only at their 

 apex, where, in both these sacs, a passage exists, called the 

 foramen (Plate Y. fig. 21. a) ; or, in the language of Mirbel, 

 exostome (fig. 25. a) in the outer integument, and endostome 

 (fig. 25. b) in the inner integument. The central part is a 

 fleshy, pointed, pulpy mass, called the nucleus (Plate V. 

 fig. 19, 20. a, 22. b, 23. c, 24. d, 25. e, 27. e). 



The outermost of the sacs (Plate V. fig. 22. c, 23. , 25. c) 

 is called the primine. It is either merely a cellular coating, 

 or it is eventually traversed by veins : these are sometimes 

 very apparent, as in Citronworts (Aurantiacese), and Mirbel 

 seems disposed to think that they often exist in a rudimentary 

 state when they are not visible. Usually it is nearly as long 

 as the secundine, but sometimes it is remarkably shorter, as 

 in Euphorbia Lathyris when very young (Plate V. fig. 22.) 



The outermost but one of the sacs (Plate V. fig. 23. b, 

 20. b, 25. d) is called the secundine ; it immediately reposes 

 upon the primine, and sometimes contracts an adhesion with 

 it, so that the two integuments become confounded. In 

 order to ascertain its existence, it is, therefore, necessary to 

 examine the ovule at a very early period of its growth. 



