PRINCIPAL GROUPS OF PLANTS. in 



not found in the Gymnosperms, but these are represented by two 

 male nuclei which are transferred directly to the archegonium 

 from the male gametophyte, formed through germination of the 

 microspore (pollen grain). It may be recalled that in the Pteri- 

 dophytes the motile sperms are discharged from the antheridium 

 and carried by the agency of water to the archegonium, but in the 

 Gymnosperms water is no longer a medium of trans ferral. The 

 microspores themselves are carried to the ovules usually through 

 the agency of wind, after which they germinate, developing a tube 

 which carries the male nuclei directly to the archegonium without 

 their ever having been free. 



The transferral of the microspores or pollen grains to the 

 ovule is known as pollination. After pollination the wall-cell de- 

 velops a tube, the pollen tube, and the generative cell gives rise 

 to two male nuclei, which, with the remaining protoplasmic con- 

 tents of the antheridium, are carried by the pollen tube to the 

 micropyle, which it enters, penetrating the tissue of the nucellus 

 (Fig. 70, t) . On reaching the neck of an archegonium the pollen 

 tube pushes its way down into the venter, where it discharges 

 one of the sperm nuclei, which unites with the egg, forming an 

 oospore. Cessation in growth does not yet take place and the 

 oospore develops into the embryo already described. The develop- 

 ing embryo obtains its nourishment by means of a suspensor 

 (Fig. 71, s), which also places the embryo in a favorable position. 



There being several archegonia in an ovule (Figs. 70, 71), a 

 corresponding number of embryos may be formed, but rarely 

 more than one survives. While the embryo is developing, the 

 other tissues of the megaspore are likewise undergoing changes 

 leading to the maturity of the seed. The carpels and seminifer- 

 ous scales also continue to grow, and they usually become more or 

 less woody, forming the characteristic cones of the pines (Fig. 

 72), but may coalesce and become fleshy, producing the berry-like 

 fruits of Juniper (Fig. 75). The seed on germination gives rise 

 to the sporophyte (tree). 



Groups of Gymnosperms.- -There are several important 

 groups of Gymnosperms: (i) The Cycads or Fern Palms, 

 which are characteristic of tropical and sub-tropical countries. 

 The trunk does not branch as in the ordinary evergreens, and 



