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COOK 



fore the conjugation is completed, and are thus a joint or conju- 

 gate product of the two germ-cells. 



The sexuality of the higher plants, known to the ancients, and 

 to the aborigines of tropical America, reasserted by Bacon, re- 

 discovered by Sprengel and substantiated by Muller and Darwin, 

 has been denied on technical grounds by recent botanical 

 writers, as a result of the prevalence of certain morphological 

 theories of alternation of generations. This doctrine has led to 

 the inference that the bodies of our higher flowering plants 

 represent an "asexual generation," and it is held to be absurd 

 to ascribe to such organisms the qualities and specializations of 

 sexuality. 



Some botanists accordingly refuse to call the stamens and 

 pistils sexual structures, or the staminate and pistillate plants 

 male and female, because they do not represent the same 

 kind or stage of sexual differentiation as that shown in male 

 and female moss-plants or male and female fern-prothallia. 

 The fact remains, however, that the sexuality of such a plant 

 as the date palm is completely analogous to the sexuality of the 

 higher animals and of man himself. In other words, it has 

 been proposed to deny sexuality to exactly that form of sex- 

 differentiation to which the word was originally applied. 



The significant fact is that the sexual differentiation of 

 organisms should have taken place on the two different planes 

 of structural organization, both in the simple-celled lower types 

 and in the conjugate-celled higher types. Indeed, there are 

 three grades or stages of development where sexual diversifica- 

 tion has taken place. 



i. Sexual differences of the single gametic cells, as of the 

 sperms and ova, or the pollen-grains and the egg-cells. 



2. Sexual differences of simple-celled gamete-bearing struc- 

 tures, as of the male and female thalli of liverworts, the male 

 and female plants of mosses, and the male and female pro- 

 thallia of ferns, Isoetes, Selaginella and Equisetum. 



3. Sexual differences of double-celled or conjugate struc- 

 tures, as of the male and female individuals of the higher plants 

 and animals. 



Nor does the reckoning end here, for the separation and 



