388 THE SPERMATOPHYTES 



364. The development of the flower. The development of the 

 parts of a flower would be expected to progress in the order of 

 sepals, petals, stamens, and carpels, for this is, of course, the 

 order of their position on the flower stalk, beginning from below. 

 The parts of man} 7 flowers do arise in this order, but there are 

 often irregularities due to the delayed appearance of some organs. 

 For example, in the shepherd's purse the petals are formed last, 

 arising between the sepals and carpels when the latter are far 

 along in their development (Fig. 310, C,p). 



The carpels are clearly separate in the beginning (Fig. 310, 

 C, D, c), and the ovules at first may be exposed on their surface 

 (Fig. 310, E), but sooner or later the carpels unite above, so that 

 the ovules are finally contained in the ovule case (ovary). 



A study of flower development makes clear the significance of 

 perigyny and epigyny (Sec. 157), for it shows that the apparent 

 fusion of parts, frequently called coalescence, when sepals, petals, 

 or stamens seem to be united to one another or to the carpels 

 (see diagrams, Fig. 136), is due to the formation of tubular out- 

 growths from zones of tissue below the floral parts. The parts 

 which are most frequently affected by these zonal outgrowths 

 are the carpels, and it seems probable that the compound pistil 

 mav have arisen from their activities. In many cases the ovules 



\j t/ 



are developed from tissue that is probably really a part of the 

 tip of the flower stalk. 



365. The life history of an angiosperm. The formula for 

 the life history of an angiosperm is the same as that of a gym- 

 nosperm (Sec. 356). The gametophyte phases, however, occupy 

 generally a much shorter period, so that the seeds are matured 

 in the same season and sometimes within a few weeks after 

 pollination. 



The formula is then as follows : 



pollen grain Male Gametophyte sperm nucleus ^ 



SpOrO- / (microspore) (protoplasmic contents \ 



< of pollen tube) > 



nhyte \ / 6tc 



\ embryo sac Female Gametophyte egg 



(megaspore) (protoplasmic contents 



of embryo sac) 



