1902] A MORPHOLOGICAL STUDY OF ASCLEPIADACEAE 395 



The hood and horn are known to enclose the nectar, but the 

 part played by the horn in pollination is not clear. 



The formation of the anthers, with their wing-like projections 

 and two sporangia, as well as the development of the microspores, 

 has been worked out by the writers referred to above. In general, 

 the account is the same for all the species examined. 



The development of the male gametophyte is pretty well 

 known. Corry (5) saw the generative and tube nuclei in A, Cor- 

 nuti, but supposed the larger to be the generative cell, Stras- 

 burger (14) found the small one to be the generative cell, and 

 the writer (7) found the same in A. tuberosa. Gager (8) observed 

 the first division of the microspore in A. Cormiti^ and the division 

 of the generative nucleus. 



A. vcrticillata proved to be the best form for the study of the 

 pollen grain, and hence it is here taken as the type, although the 

 others agree with it in general features. The generative cell, 

 flattened out against the wall usually in lenticular or hemispheri- 

 cal form {^fig, 22) and varying in size, soon loosens and approaches 

 the tube nucleus {fig. 2j). The two nuclei are greatly dif- 

 ferent in size and both have the chromatin in granular threads. 

 The generative nucleus usually has massed about it a quantity 

 of denser cytoplasm of finer structure and bounded by a definite 

 wall. The wall sometimes persists until division is complete 

 {fig. 26), and at other times division follows its resorption (figs, 

 24, 2s), The lenticular form is gradually changed to crescentic, 

 oval, or even spherical. The division forming two male ce 

 occurs about the time the buds open. It is not quite simulta- 

 neous in the same anther, although the amount of variation is not 

 great. In all cases observed, division occurs near the tube 

 nucleus. In fig. 25 eight chromosomes were counted, but not 

 with absolute certainty. Those in the vegetative cells of the 

 stamens are about the same in number, contrary to expectation, 

 but they are so extremely small that a definite statement is not 

 warranted. The condition of the cytoplasm in the pollen grains 

 of the same anther seems to vary. In some it is reticulated, in 

 others homogeneous. In almost all one finds groups of the 

 deeply staining bodies of undetermined nature {fig- 24). In 



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