308 COLLEGE BOTANY 



raiigia j on the under surface of each. They contain great quan- 

 tities of pollen (or microspores ) which escapes and is carried by 

 the wind, some of it falling into the open pistillate cones, where 

 it comes directly in contact with the ovules (i.e., pollination). 

 After pollination the macrosporophylls close, forming a compact 

 cone (Figs. 152, 153). 



The microspores consist of single cells, each with two ex- 

 panded wings which facilitate their transportation by the wind. 

 The divisions of the single-celled microspore gives rise to a very 

 small, few-celled male prothallium (or Gametophyte) bearing 

 two sperms. These sperms are not motile and never escape as 

 in the preceding groups, but pass down, a tube formed by a growth 

 of the male prothallium until they reach the archegonium, when 

 fertilization occurs as in the preceding groups. The fertilized 

 cell is a young Sporophyte (or embryo)' and is surrounded by 

 parts of the female Gametophyte, the nucellus and the integu- 

 ments. In other words, we now have what is known as a " seed " 

 composed of the parts just mentioned. The integuments produce 

 a wing-like structure by which the mature ><< d can be readily 

 can-led by the wind (Fig. 153). When the seeds are mature, 

 the macrosporophylls spread apart and permit them to fall out. 



In order to get a connected understanding of the relationship 

 of the various groups of plants we should now review Chapter VI 

 on the flower, giving special attention to the part on reproduction. 

 The carpels of the pistil are the macrosporophylls and the ovule 

 (or more strictly the nucellus) is the macrosporanf/iinn. One 

 cell (occasionally two or more) of the macrosporangium becomes 

 more prominent than the surrounding cells and is readily recog- 

 nized by its size and the rich character of its contents. The 

 future history of this cell varies in different species of plants, 

 but in most cases it eventually gives rise to four cells which arc 

 known as the mega-spores. In most cases only one megaspore 

 survives and that one is usually the one farthest from the micro- 

 pyle. It enlarges very rapidly, absorbing the other three mega- 

 spores and some of the cells of the nucellus. The nucleus divides 



