Septembek 23, 1898.] 



SCIENCE. 



385 



gametophyte is supplied with outside nour- 

 ishment by the retention of the megaspore, 

 is hinted at among the heterosporous 

 pteridophytes and gymnosperms. These 

 tendencies have found full expression in 

 the angiosperms, where archegonia have 

 disappeared and the reproductive tissue 

 of the female gametophyte is persistently 

 organized before the nutritive tissue. Evi- 

 dence as to the details of the evolution of 

 this tendency is lacking and may not be in 

 existence, but the tendency has certainly 

 reached a remarkably definite expression. 

 The unvaried appearance and movement of 

 eight free nuclei or cells, and the remarka- 

 ble fusion of two of them, represent habits 

 so fixed through such an enormous group 

 that they baffle explanation, and argue 

 both for the monophyletic origin of angio- 

 sperms, and against their derivation from 

 so divergent a line as gymnosperms. 



The earlier evolution of the gymnosperm 

 line is probably to be explained by ecolog- 

 ical conditions. The body as a rule is 

 organized to endure extreme conditions. It 

 is certainly not a mesophytic type, and its 

 evolution was certainly not in response to 

 prevailing mesophytic conditions. On the 

 contrary, the angiosperm type is essentially 

 a mesophytic one, with great foliage display, 

 and probably expanded in response to 

 widely prevalent mesophytic conditions. 

 This might explain the habit peculiarities 

 of the two groups, but whether the more 

 recondite morphological differences hold 

 any relation to these or not is too obscure 

 to permit even speculation. 



SUMMARY. 



1. A great Cordaites plexus, more exten- 

 sive than the one usually included under 

 that name, represented the characteristic 

 Palseozoic seed plants. 



2. It was probably derived from homos- 

 porous-eusporangiate Filicales, represented 

 to-day most abundantly by the Marattia 



forms and their allies, and was the most 

 common Palaeozoic type of Filicales. 



3. From it the gymnosperm lines, at least 

 the cycads and conifers, were derived, the 

 usually recognized Cordaites representing a 

 transition stage towards conifers. 



4. The frequent independent appearance 

 of heterospory is to be expected, as it prob- 

 ably results from inequalities of nutrition 

 in connection with the development of 

 antheridia and archegonia. 



5. The retention of the megaspore, result- 

 ing in the seed habit, follows the extreme 

 sterilization of the megasporangium, which 

 is attained with the organization of but one 

 megaspore. "With the development of a 

 single megaspore imbedded in sterile tissue 

 shedding becomes mechanically difficult, 

 unnecessary, and even disadvantageous from 

 the standpoint of nutrition. 



6. The retention of the megaspore was fol- 

 lowed by the development of seed coats, 

 possibly through the well-known effect 

 of fertilization upon adjacent tissues ; by 

 immediate germination of the oospore, on 

 account of the favorable conditions and the 

 abundant supply of available nutrition ; and 

 by the checking of the developing embryo 

 by the mature seed structures, resulting in 

 the characteristic intra-seminal and extra- 

 seminal stages of germination. 



7. The first retained megaspores were 

 doubtless directly exposed to the micro- 

 spores, and in Cordaites and cycads a pollen 

 chamber of varying depth and extent is 

 associated with the early stages of sipho- 

 nogamy, with which the spermatozoid habit 

 was more or less associated. 



8. The pollination of gymnosperms is but 

 a continuation of the ordinary method of 

 dispersing aerial spores employed by cryp- 

 togams, the chief result of the retention of 

 the megaspore upon the male gametophyte 

 being the development of siphonogamy. 



John M. Coulter. 

 University of Chicago. 



