34$ POLLINATION 



is partially fused on one side with the ovuliferous scale (Fig. 202, 

 F), and, except in a few genera, the central nucellus is only free 

 from the integument in the region of the micropyle (Fig. 203, 

 B, mi.). A single megaspore (p.) is usually differentiated within 

 the nucellus of the young ovule by the division of a mother-cell 

 which arises hypodermally. This forms a linear tetrad of 

 potential spores of which only the innermost survives. The 

 functioning spore rapidly enlarges and eventually displaces 

 the greater part of the nucellus, though a pronounced cap 

 of tissue still remains in the free portion beneath the micro- 

 pyle (Fig. 203, n.). The thin-walled parenchymatous female 

 prothallus (p.), which is richly supplied with food-material, 

 as a rule arises only after pollination has occurred. The 

 archegonia (ar.}, which are usually few in number, are generally 

 differentiated at the micropylar end. Each consists of a rela- 

 tively huge ovum (o.) embedded in the tissue of the prothallus, a 

 minute ventral canal cell (v.c.), and a very short inconspicuous 

 neck (a.) composed of one or more tiers of cells. When the 

 archegonia are numerous (as in most Cupressinese) they are 

 usually collected together in groups, opening into a common 

 shallow depression (archegonial chamber), at the front end of the 

 prothallus. 



When the mature pollen is being scattered broadcast by the 

 wind, the axis of the female cone elongates slightly, so that the 

 cone-scales spread apart, thus giving access to the ovules. At 

 this time each of the latter secretes a mucilaginous drop which 

 oozes out from the micropyle, and serves to catch up the micro- 

 spores as they are blown between the scales. By the gradual 

 shrinking of the mucilage the pollen grains are sucked through 

 the micropyle on to the surface of the nucellar cap, and, soon 

 after this, the scales of the female cone enlarge and once again 

 fit tightly together. This process of pollination must be sharply 

 distinguished from that of fertilisation which occurs at a later 

 stage often only after a prolonged interval (see below). 



When shed, the contents of the microspores have already under- 

 gone division, the cell, or cells, thus cut off .from the general body 

 lying against one side of the grain (Fig. 204, B) ; in most cases 

 this comprises the single antheridial cell (a.c.) only, but in Pinus 

 and related forms its formation is preceded by the cutting off 



