218 Trans. Acad. Sci. of St. Louis. 



SUMMARY. 



Structurally there are wide differences between the tribes 

 of the order, very marked in some closely allied genera. 

 The pericarp is well developed in such genera as Zea, Arundi- 

 naria, and fairly well in Trilicum, Secale, Hordeum and Avena. 

 The testa is but slightly developed in most cases, notably 

 so in Fesluca, Panicum glabrum, Aristida and Oryza saliva, 

 the protective features being provided for by the glumes 

 surrounding the fruit, or the wall of the ovary. The nucellus 

 is never entirely absent, especially in the groove. It is usually 

 much compressed. In the genera Fesluca and Bromus, the 

 cells are large, thick-walled and mucilaginous, and no doubt 

 act as reserve food. The aleurone layer is variable. It is never 

 absent. Of one row of cells in Trilicum, Zea, Zizania. The 

 cells are very small in Panicum Crus-galli, Aristida, Setaria 

 italica. Of more than one row of cells in Avena, Arrhena- 

 therum, Fesluca, Hordeum vulgare. The starch cells differ in 

 size and contain small spherical or elliptical grains in Sor- 

 ghum vulgare and Cenchrus tribuloides. Large spherical or 

 somewhat elliptical grains occur in Trilicum and Hordeum, 

 accompanied by numerous smaller ones. Small five or six- 

 sided grains in Panicum Crus-galli, Zea Mays, Euchlaena 

 mexicana. This applies in general to the tribe Maydeae and 

 Paniceae. Cenchrus is, however, an exception to the rule. 

 Compound starch grains occur in Zizania, Oryza, Avena, 

 Arrhenalherum, Glyceria, Poa, Phalaris and Arundina- 

 ria; most grasses appear to have compound grains. The 

 endosperm always contains protein, though much reduced in 

 the starch cells, except in the aleurone layer, where no starch 

 occurs. The starch cells next to the endosperm contain more 

 protein than the interior of the endosperm. Fat is also 

 present in small amounts. The compound starch grains of 

 Nardus in the tribe Hordeae are somewhat anomalous. 



