778 PHYSIOLOGY. 



composed of several elongated cells, which are doubtless pro- 

 duced by the ordinary process of cell-division. The time required 

 for the development of these tubes also varies in different pollens ; 

 thus, sometimes they are developed almost immediately the pol- 

 len comes into contact with the stigma, whilst in other cases, 

 many hours are required for the purpose. The pollen-tubes also 

 occupy a varying time in traversing the canal of the style — that 

 is, from a few hours to some weeks. When the pollen-tubes 

 have penetrated the stigmatie tissue, the secretion of the latter 

 ceases and the stigma withers. The upper part of the pollen- 

 tubes also wither above, as growth takes place below. 



The pollen-tubes having reached the ovary are distributed 

 to the placenta or placentas, and then come into contact with 

 the ovule or ovules. One (or sometimes two) of these pollen- 

 tubes enters into the micropyle of each of the ovules {figs. 

 1123 and 1124), and thus reaches the nucleus and embryo-sac. 

 When it arrives at the latter it is generally somewhat en- 

 larged {figs. 1123, t, and 1124, t), and adheres firmly to it at 

 or near its apex. The embryo-sac is freqiiently introverted to a 

 slight extent at the point of contact with the pollen-tube {figs. 

 1123 and 1124), and it is stated by Hofmeister, to perforate it in 

 Caniia, but if such a perforation occurs in this case, it is alto- 

 gether an exception to what is generally observed. As soon as 

 contact of the pollen-tube and embryo- sac is effected, a kind of 

 osmotic action between the contents of the two takes place, the 

 result of which is the development of one, or rarely two, as in 

 Orchis and Citrus, or more, of the germinal vesicles, into em- 

 bryos. According to Henfrey, as previously noticed (see p. 776), 

 the first change is the development of the germinal corpuscle 

 into a germinal vesicle or cell. 



The germinal vesicle, in its development into an embryo, gene- 

 rally divides in a transverse manner into two cells {fig. 1124, e); 

 the upper of which by elongating, and frequently by further 

 division, forms a suspensor {fig. 1123, s), by which the lower cell 

 is suspended from the apex of the embryo-sac. This lower cell 

 assumes commonly a globular form {fig. 1123, r), and ulti- 

 mately by cell-division forms the embryo, whether mono- or 

 di-cotyledonous. The suspensor is not present in all cases, 

 while in others where it is found it varies in length. It is 

 evidently not essential in all instances, as it always shrivels up 

 during the development of the cell which it supports into the 

 embryo. The latter, therefore, is the true rudimentary embryo. 

 Other variations occur in the mode in which the germinal vesicle 

 is developed into an embryo, but the above is a general sketch 

 of the subject, and all that our space will allow us to give. 



The changes which take place in the ovule during the deve- 

 lopment of the embryo, and the subsequent growth of the latter, 

 have been already alluded to when treating of the seed. (See 



