372 



PART IV. VEGETABLE TAXONOMY. 



imperfectly developed, and in this species the process of fertiliza- 

 tion occupies a long time, not being completed, in fact, until the 

 succeeding year. 



When the ovule is fully matured and ready to receive the 

 fertilizing influence of the pollen-tube, a large embryo-sac has 

 been formed in the nucellus, and this has been filled with cells 

 constituting a prothallium, and in this are formed one or more 

 archegonia, which structurally resemble the corresponding organs 



Fig. 572. — Pinus sylvestris. A, branch, showing cluster of staminate cones near its 

 apex ; a, one of the staminal leaves magnified, showing two pollen-sacs ; b, a pollen-grain 

 highly magnified, showing outer coat with two cellular, wing-like appendages. The body 

 of the grain contains two cells, one large and one small one. 



of the Pteridophytes, possessing a body, which contains a germ- 

 cell or oosphere, and a neck composed of rows of small cells. 

 The pollen-tube, formed from the germinating pollen-grain, pene- 

 trates the tissues of the nucellus and grows down through them 

 to the neck of the archegonium, and its protoplasm mingles with 

 that of the oosphere. In some species the nucleus of the one 

 has been observed to blend with that of the other. The fertil- 

 ized oosphere then begins to develop in its interior an elongated 

 and commonly more or less contorted mass of cells, called the 



