202 Plant Biology 



Pine trees have large, branched stems, and the needlelike leaves are 

 borne in clusters on short, spurlike branches. The number of leaves per 

 cluster (2 to 5) and the length of the leaves vary with the species. 



In the pines, the male and female cones are borne on the same tree 

 (monecious) (Fig. 52). In other conifers, the male and female cones 

 may be borne on separate plants (diecious). The simple, staminate 

 (male) cones are smaller than the female and are home in a group. 

 Each male cone is composed of microsporophylls which are spirally 

 arranged and attached to a central axis. Each microsporophyll bears 

 two microsporangia on the undersurface, in which are produced numer- 

 ous microspore mother cells, each of which produces four m,icrospores 

 (pollen grains). Each pollen grain develops into a microgametophyte 

 by producing two prothallial cells and an antheridial cell. The latter 

 divides to form a generative cell and a tube cell. During this process a 

 pair of ''wings" forms on the four-celled pollen grain to assist in its dis- 

 semination by the wind, in some cases for hundreds of miles. 



The ovulate (female) cones are larger than the male and usually are 

 borne singly. Each female cone (Fig. 52) is composed of scalelike mega- 

 sporophylls attached to a central axis. Each megasporophyll bears two 

 ovules on its upper surface. An ovule consists of (1) an external, pro- 

 tective integument which has a micropyle (small opening) for the en- 

 trance of a pollen grain and (2) a central megasporangium. (nucellus). 

 When young, the megasporangium has one megaspore mother cell which 

 produces four megaspores, three of which abort. The one megaspore 

 develops into the female megagametophyte which contains two or three 

 archegonia. Each archegonium contains an egg within a venter. 



During pollination the wind carries the pollen grains (microspores) 

 to the female cones, where pollen enters through the micropyle and con- 

 tacts the megasporangium (nucellus) by means of a sticky liquid. The 

 pollen grains form pollen tubes through the nucellus toward the arche- 

 gonia. The generative cell and the tube cell pass through the pollen 

 tube, and the former produces two, nonmotile sperms (m,ale nuclei). 



About one year after pollination the fertilization process occurs and 

 consists of the fusion of one sperm with the ^^cr within the archegonium. 

 The resulting zygote, by cell division, eventually produces an embryo and 

 suspensor cells. The latter force the embryo in contact with the food 

 endosperm (transformed female megagametophyte). Later the embryo 

 develops an epicotyl (epi-kot'il) (Gr. epi, upon; kotyle, vase or cup) 

 and a hypocotyl (hi po -kot' il) (Gr. hypo, under), which bears a number 

 of primary, embryonic seed leaves known as cotyledons (kot i -le' don) 



