174 MORPHOLOGY OF THE ANGIOSPERMS 



the higher insects arose. Angiosperms, with well-defined nectaries and 

 stigmas, and higher groups of insects, with hairy bodies, perhaps de- 

 veloped together in Jurassic and Cretaceous times. The closure of the 

 stigma canal and of the carpel itself have also been considered an 

 accompaniment of pollination by the higher insects. 



Entomophilous flowers that are visited by bees, moths, and butterflies, 

 have conspicuous perianth, fragrance, and nectar glands. Flies visit 

 chiefly flowers with disagreeable odor and reduced or dully-colored 

 perianth. Flowers pollinated by birds are usually large or closely 

 grouped in large, conspicuous inflorescences, and secrete quantities 

 of nectar. The large, bird-pollinated inflorescences of the Australian 

 waratah, Telopea speciosissima, and some species of GrevUlea drip 

 with glistening nectar, even under dry atmospheric conditions. The 

 highly colored organs in Telopea are the bracts and calyces; in GrevUlea, 

 the calyces. The nectar is secreted by glands that represent vestigial 

 petals in both of these genera. 



The pollen grains of wind-pollinated plants are small, light, smooth, 

 and usually nonadhesive, not cohering in clusters; those of entomophi- 

 lous and ornithophilous plants are strongly adhesive, borne in pollinia, 

 tetrads, or free but cohering in masses. The pollen grains of genera 

 pollinated under water are elongate — filamentous, or confervoid and 

 not adhesive. Vestiges of adhesive material have been found on the 

 wind-borne pollen of taxa with close entomophilous relatives: Acer 

 ( anemophilous species), Riimex, Ambrosia, Sanguisorba. Pollen type, 

 method of pollen distribution, and structure of the gynoecium are, 

 biologically, closely correlated. 



THE MALE GAMETOPHYTE 



The newly formed microspore has a large, centrally placed nucleus, 

 dense cytoplasm, and a delicate wall. Before germination, it increases 

 greatly in size and volume (Figs. 70B and 71F). A central vacuole de- 

 velops, and the nucleus takes a peripheral position. UsuaUy germina- 

 tion occurs only some days or weeks after the spore is mature, though, 

 in tropical plants, it may be immediate. In temperate-climate plants 

 that flower in the spring, the sporangenous tissue may overwinter at 

 the spore-mother-cell stage, the mature microspore stage, or at the two- 

 cell stage of the gametophyte; perhaps more taxa overwinter at the 

 spore-mother-cell stage than at later stages. Germination of the spore 

 begins with cell division and the formation of a small, walled cell, which 

 lies against the spore wall (Figs. 70D and 71G). This first cell of the 

 gametophyte is rarely formed centrally in the spore by "free cell forma- 

 tion," development without typical wall formation; the phragmoplast 

 forms a delicate cytoplasmic "wall," which delimits the protoplast — 



