240 



General Biology 



in length. There is a general axis on which flat megasporophyls are 

 borne. Each of these megasporophyls bears two inverted megasporangia 

 or ovules (Fig. 144). 



The pollen falls between the megasporophyls (called carpels in the 

 flowering plants), and each microspore then pushes out a pollen tube 

 which penetrates the ovule tissue. This process stimulates the growth 

 of the cone tissues, and the cone, therefore, increases in size. The ovules 

 also enlarge, and the upper end of the ovule develops a thickened mass 

 of green tissue which grows beyond the end of the sporophyl, to form 

 the seed scale. These seed scales are merely the distal ends of the ovules, 

 and function as organs of photosynthesis for a year or so. 



Fig. 143. Scotch Pine (Pinus sylvestris). 



A-D, stages in the development of the carpellate cone, and its car- 

 potropic movements. E, very young carpellate cone much enlarged; F, 

 vetral, G, dorsal views of a scale from E; 1, ovuliferous scale; 2, ovule 

 (in logitudinal section); 3, pollen chamber and micropyle leading to 

 the apex of the nucellus (megasporangium) ; 4, integument of the ovule; 

 G, 1, tip of ovuliferous scale; 5, bract; 4, integument; H, longitudinal 

 section at right angles to the surface of the ovuliferous scale (diagram- 

 matic) ; 6, megaspore; 7, pollen chamber, /, longitudinal section of a 

 mature cone; 6, ovule; J, scale from a mature cone; 6, seed; w, wing of 

 seed; K, dissection of mature seed; h, hard seed coat; c, dry mem- 

 branous remains of the nucellus, here folded back to show the endosperm 

 and embryo; e, embryo; p, remains of nucellus; L, embryo; c, coty- 

 ledons; e, hypocotyl; r, root-end. (From _ C. Stuart Gager's "Funda- 

 mentals of Botany," by permission of P. Blakiston's Son & Co., Publishers.) 



The following summer or autumn a spore-mother-cell, also known 

 as an archespore, arises in the interior tissues of the ovule. This arche- 

 spore then divides into four cells which are really four young mega- 

 spores, although only the one lying in the lowest position actually de- 

 velops into a full-fledged megaspore. 



This megaspore then divides and subdivides until a rather solid 



