170 Sargant and Arber.—The Comparative Morphology of 
I. Comparative Anatomy of Grass Seedlings. 
A. Avena type. 
Avena sativa , L. The structure of the embryo within the seed is 
well known. Only such details as are important for our purpose need be 
described here. A median section is outlined in Text-fig. i, p. 161. 
The scutellum is fleshy, but leaf-like in outline. It has a well-developed 
dorsal ridge (Text-fig. 13), and a ventral scale (Text-fig. 14). Whatever 
the morphological nature of this scale may be, its function is quite clear. 
The endosperm—-which encloses the upper part of the scutellum like a cap— 
fits into the furrow between scutellum and scale, and ends there. Thus the 
plumule, which lies immediately below the scale, is quite clear of the endo¬ 
sperm, and is not impeded by it when growth begins. The scale is at some 
Text-figs. 9-11. Avena sativa, L. 9. Whole seedling, life-size. 10. Same seedling with husks 
of grain removed, n. Same seedling with scutellum laid bare. 
distance above the insertion of the scutellum on the axis, and the scutellum 
itself is prolonged below its insertion. 
The first region of the axis to elongate is the mesocotyl, but the 
primary root is tlie first member to show externally in the germinating 
grain. In Text-fig. 9, the primary root is an inch and a half long, while the 
tip of the stem-bud can hardly be perceived among the glumes. Two cauline 
roots from the zone of insertion are also well developed. 
The mesocotyl bears the stem-bud enclosed in the coleoptile, a stiff 
conical sheath with a sharp point. In terrestrial Grasses the coleoptile 
serves to force a way upwards through the soil, while protecting the stem- 
bud within it. 
The mesocotyl may continue to grow for a considerable time. When 
grains are scattered on the ground in the usual way, the ascending axis 
of each grows upwards as soon as it can get clear of the glumes, making an 
angle of about 90° with the long axis of the grain. The length of the 
mesocotyl is then very variable. Growth appears to cease when the base 
