GENERATION". 137 



straight and direct, though sometimes it twists and turns 

 about very remarkably. 



231. In this passage of the pollen-tube downwards 

 from the stigma, the object of it has been to come into 

 contact with the ovule; this the end of the tube at 

 length meets with, and having done so it enters it by 

 the foramen ; and this being accomplished, it pierces the 

 point of the nucleus. 



232. The nucleus, in most plants, is a conical body 

 formed of a mass of cellular tissue, whilst in some it ap- 

 pears as a hollow cellular skin or case. 



233. When the nucleus is composed of a solid mass 

 of tissue, about the time when the pollen comes into 

 contact with the stigma that is as a general rule a 

 vacuity or hollow appears within it at some little distance 

 from its upper end. This enlarges, proceeding down- 

 wards along the axis of the nucleus, whose tissue gra- 

 dually more or less disappears, a new membrane is 

 formed, which now encloses the enlarged hollow, and 

 we have formed the sac of the embryo, or quintine of 

 some writers. 



234. In most cases the sac of the embryo is com- 

 pletely developed by the time the pollen-tube pierces 

 the nucleus, at least at its upper or micropyle end 

 and which is the part coming into contact with the 

 tube which is entering ; but sometimes the sac is 

 not begun to be formed until the pollen- tube has 

 already reached the nucleus. 



235. The pollen-tube, having entered the ovule by the 

 foramen, and pierced the nucleus at it upper part, comes 

 in contact with the sac of the embryo ; at the upper 

 part of this sac, according to Brongniart, a transparent 

 little bladder, containing minute granules, is then to be 

 seen, which is open at one point, and seems to have had 

 its origin in a pressing inwards of the membrane of the 

 sac of the embryo. In this bladder, after a time, a 

 cellular mass is formed, and our present knowledge (says 

 Meyen) upon this subject satisfies us, that this bladder 



N 3 



