102 A. H. Graves, 



floral axis is already well developed, one finds a correspondingly 

 advanced development in the floral scale leaf, showing that it is 

 now connected with and influenced by the flower and peduncle. 



5. Morphological and Physiological Nature of Scale Leaves. 



That the vegetative scale leaf is a leaf and not an axillary stipule, 

 may be accepted without question, for its development occurs much 

 later than that of the subtending leaf and its origin is from the 

 axillary bud of the latter. As to whether one has to deal here with 

 a degenerate leaf, or one that approximates the primitive form is a 

 question difficult to settle without a definite knowledge of the 

 primitive monocotyledonous leaf form. It is probable, however,^ 

 that the ordinary foliage leaves are nearer to the primitive form. 

 Moreover, the lack of vascular tissue and chlorophyll, and the 

 simple structure in general indicate a degenerate form. 



Whatever its genetic origin, the early development and complete 

 enfolding of the young primordia, which later grow out beyond it, 

 point to the present function of the vegetative scale leaf as being 

 essentially protective. 



The nature of the floral scale leaf is not so clear. The striking- 

 fact, however, that the branch adjacent to this scale leaf never has 

 a vegetative scale leaf at its base would seem to point to the con- 

 clusion that the floral scale leaf is simply this vegetative scale leaf 

 now connected with the flower and utilized for its protection in an 

 embryonic state. 



6. Comparison with Other Potamogetonaceae. 



• Scale leaves (prefeuilles) are found in Althenia (Prillieux, 1864)' 

 where there are often two on a branch below the first ordinary 

 foliage leaf. They occur also in Zannichellia (Irmisch, 1858, p. 30) 

 and Cymodocea (Bornet, 1864). In Potamogeton (Irmisch, 1858) the 

 scale leaves are quite abundant, occurring on the rootstock as well 

 as at the base of the shoots. The prophyllon, or fore-leaf, described 

 by Holm (1905) in his writings on the grasses, is an homologous 

 structure. 



Root 



A. General Characters 



The roots in Ruppia, as in many submerged plants, are simple, 

 and strikingly reduced in structure. The primary root, formed 



