122 A TEXT-BOOK OF GRASSES 



The student should take careful note of the theoretical relations 

 of the parts of the spikelet, since it is a knowledge of these relations 

 that enables one to assign a morphological status to an absent organ. 

 The glumes and lemmas are morphologically equivalent, namely 

 bracts. But in the great majority of species of grasses the lower 2 

 bracts of the spikelet are empty and the others above contain flowers. 

 By definition the lower pair are called glumes and those above are 

 called lemmas. The glumes are nearly always differentiated structu- 

 rally from the lemmas. 



The theory of the evolution of organisms teaches us to trace 

 the development, progressively or retrogressively, of organs through 

 groups of allied species. Such an examination will usually enable us 

 to interpret correctly the morphology of the organs. For example, we 

 wish to know the morphology of the spikelet of Reimarochloa and 

 Homalocenchrus. In the former we have a spikelet consisting of 1 

 empty bract and 1 flowering bract. How is this to be interpreted? 

 In the first place we are confident that the genus belongs to the large 

 tribe Panicea^ and that it is closely allied to Paspalum and Panicum. 

 The typical spikelet of the Panicese consists of 4 bracts, the upper- 

 most of which contains a perfect flower. This bract, by definition 

 a fertile lemma, is distinctly different from those below. The first 

 and second bracts are empty and by definition are glumes. The third 

 is by definition also a lemma even though it contains no flower. An 

 examination of the spikelets of various genera shows that there are 

 all gradations between species in which the lower lemma, usually 

 called the sterile lemma, contains a perfect flower (Isachne) to those 

 which contain stamens, or only a pale a, and finally to those which 

 are empty. This, of course, confirms the statement that the third 

 bract is a lemma. No transitions are found between the glumes and 

 the lemma. But we do find a tendency on the part of the glumes to 

 retrogress in size. The first glume is usually smaller than the second, 

 and the retrogression can easily be traced through its slight develop- 

 ment in Syntherisma and Panicum to its disappearance in Paspalum. 

 Similarly the second glume shows a tendency to disappear, cul- 

 minating in its absence in Reimarochloa. Furthermore, there is 

 no tendency for the second glume to disappear before the first. From 

 the above we conclude that the single empty bract below the fertile 

 lemma in Reimarochloa is the sterile lemma, that is, it is homologous 

 with the third bract or sterile lemma of the typical spikelet of the 

 tribe. 



