44 FERTILISATION. 



other by a thin septum, and each is provided with a nucleus. It is the 

 larger of the two that grows out into a pollen-tube when the pollen 

 grains are brought into contact with the ovule ; the outer cell wall, the 

 extine, ruptures, and the thicker inner wall, the intine, presses through 

 the chink in the form of a teat-like outgrowth which lengthens into 

 a pollen- tube ; the whole contents of the larger pollen-cell effuses 

 gradually into the pollen-tube ; the nucleus shifts to the end of the 

 pollen-tube which pushes its way through the tissues of the ovule into 

 the embryo-sac. In Pinus the pollen grains have, in addition, two 

 small outswellings, one on each side, filled with air which diminish 

 considerably the relative weight of the grain and act as wings for 

 its transport through the air; these wing-like appendages arise from 

 the outer layer of the cell- wall and increase rapidly in size when the 

 pollen is ripe. 



The ovules at the time of pollination consist of small masses of 

 spongy tissue which are thus differentiated : There is the nucleus or 

 embryo-sac* in which the ooplasm or egg-cell is embedded ; joined to 



this on the under side are several cell-layers 

 that form an enveloping sheath, whilst on 

 the upper side is a funnel-shaped opening 

 called the micropyle on which the pollen 

 grains fall. In the Abietinese the micropyle 

 is turned away from the free margin of the 

 ovuliferous scale, in the Cupressineae it is 

 turned towards it. The nucellus or embryo- 

 sac just before fertilisation becomes filled 

 with a tissue called the endosperm, and 

 produces at its apical end egg-cells which 

 vary in number in the different tribes from 

 two to fifteen, and which are always in 

 close proximity to each other beneath the 

 micropyle. When pollination takes place, the 

 lining of the micropyle is rendered sticky by 

 drops of a mucilaginous fluid secreted from 



it by which the dry pollen grains are retained and afterwards drawn 

 through the micropyle, when the pollen-tube issuing from the larger 

 cell makes its way through the tissue of the endosperm, and with 

 the nucleus at its end enters one of the egg-cells and its fertilisation 

 is effected. The tissue on the under side of the ovule increases in 

 size by cell division, closes over the micropyle and ultimately forms 

 the testa or shell of the mature seed which in some species of Pinus 

 (P. Cembra, P. edulis, P. Saliniana) is so much thickened that the 

 seed resembles a nut. 



The process of fertilisation here described is that which takes place 

 in all those species whose seeds are matured in the autumn of the 

 same year in which pollination is effected. But in Pinus, Cupressus 

 (excluding the section Chamsecyparis), Juniperus and those genera in 

 which the seeds do not ripen until the second or third season after 

 pollination, the fertilisation of the egg-cell is temporarily arrested. The 

 manner in which this retardation is brought about in Pinus Laricio 

 has been carefully worked out by Dodel-Port "After pollination the 

 ovuliferous flowers, with the exception of the bracts, grow rapidly 

 * The archegonium of some authors. 



345 



Fig. 29. Abies firma. 1 and 2, anthers 



after dehiscence x 10 ; 3, 4, and 5, 



pollen grains x 120. 



