IN FLOWERING PLANTS. 157 
When completely developed it communicates both with 
the apex, and base of the nucleary cavity : its diameter be- 
ing either uniform throughout, or larger towards the apex of 
the nucleus, dwindling inferiorly into a mere thread of com- 
munication. Of this sac there are two curious modifications, 
and although the origin is in both cases the same, yet the 
growth is materially different, in the one consisting of the 
simple extension of the original simple vesicle, in the other 
consisting in the growth of additional cellules from the sur- 
face of the original simple one. 
I find the presence of albumen almost always allied to the 
existence of the first form. 
[Cucurbitacez; a strong exception. The macropodous 
forms however, are not an exception, because, in almost all 
of them, the embryonary sac contains albumen, and in 
these it is membranous; it will be interesting to ascertain 
whether in these, similar modifications of this sac are 
observable. 
In the other modification, the sac even at a very early pe- 
riod presents obvious traces of cellularity of composition. 
I believe that this sac is always distinguishable in the ripe 
seed. Although it frequently becomes intimately incorpo- 
rated with the albumen, the examination of the surface of 
this, will generally be found to reveal its existence. 
In many instances where no albumen is formed, or in other 
words, when this sac is membrano-cellular, its existence in 
the ripe seed is most conspicuous, particularly in those in 
which the nucleus adhered in the ovulum to the coat. I need 
only instance Composite, and Boraginez ; in the former of 
which, this coat has been described as albumen.* Ascle- 
piadez and Apocynez, are other instances. 
The distinction of the coat resulting from this sac in the 
ripe seed, from that resulting from the nucleus is difficult to 
draw, but its apex, although frequently similar in form to that 
of the nucleus, is never discoloured. 
* I may here up that the seeds of Composite are not, » least in a number 
of cases akenia, but rather caryopsides, the tes have been : 
able to examine, united more or less firmly to the cavity of the pericarpium. 

