1902] BRIEFER ARTICLES 22 S 



regarded as a reproductive organ in the sense in which an antheridium, 

 archegonium, or sporangium is so regarded. 



A common definition of the seed describes it as a ''ripened ovule." 

 The full value of this is appreciated when one finds the ovule defined 

 as an ** immature seed/' Seeds, however, occur in plants destitute of 

 ovules, in the ordinary sense, and in any event a thorough definition is 

 desirable. The following statement covers, to some degree, the mod- 

 ern concept of the seed: ''A seed is an ultimate, trigenerational, sym- 

 biotic unit in the plant life-history, integrated from tissues and 

 structures belonging to two sporophytic generations and the interven- 

 ing gametophytic phase/' By a somewhat vague and imperfect analogy 

 the seed might be compared with the " household " in human society. 

 Like the average household it comprises organisms linked by consan- 

 guinity and consecutive in development. 



Thus defined, seeds maybe classified into the following groups, the 

 significance of which is essentially phylogenetic. 



I. GENERAL CLASSIFICATION. 



Seeds facultative. A. Pseudosperms. 



Example, Selaginella. 

 Seeds obligatory, B, Eusperms. 



r. Embryos monomorphous. 



a, Pteridophytic seeds, I. Pteridosperms. 



Example, Lepidostrobus. 



*. Archespermic seeds. II. Archesperms. 



2. Embryos dimorphous. 



a. Metaspermic seeds, IH. Metasperms. 



In the above it will be observed that the basis for the separation of 

 seeds into the two fundamental groups is determined by their appear- 

 ance as (i) unnecessary or (2) necessary aggregates in the life of the 

 species. In by far the great majority of seed-bearing plants seeds are 

 characteristic and are normally developed as such. In Selaginella cer- 

 tain megasporangia have been observed to remain for a long time inde- 

 hiscent, and in this condition, their spores having germinated, and the 

 eggs having been fecundated, to assume the character of seeds. This 

 is, however, not the rule in the genus but the exception, hence such 

 seeds have here been named facultative seeds or Pseudosperms, to dis- 

 tinguish them from the obligatory seeds or Eusperms of plants in which 

 indehiscence of the megasporangium has become the rule, and the seed 

 is thus fixed as a normal unit in the life-history. 



