n GRAMINEiE 439 



AVENA (Oat) 



A. sativa (Oat). — The flowers are homogamous, and 

 open between 2 and 4 p.m. According to Godron the 

 stamens turn downwards before the anthers open, so 

 that the flower could not often fertilise itself Other 

 observers, however, dispute this. Kornicke found that 

 in some varieties, a few of the flowers open in the 

 morning, and the rest in the afternoon. In wet weather 

 the flowers are almost cleistogamous. 



The awns are very hygroscopic, thus moving the 

 fruits and making them seem almost alive. Hildebrand 

 explains ^ the mechanism as follows. The awn contains 

 two kinds of tissues— ( 1 ) long, thick -walled cells ; and 

 (2) more rounded parenchymatous cells. The long cells 

 form a pillar in the shape of a T, with the upper arms 

 turned down, and a very large foot. As the upper pait 

 presents a larger proportionate surface it contracts more 

 rapidly than the foot. It is probable that the twisting 

 of the awn is due to the same cause as in Erodium (see 

 ante, p. 132). 



A. fatua (Wild Oat), a common weed of cultivation 

 in all corn countries, has been regarded as the original 

 form of the cultivated Oats. It is annual, while our 

 two other British species, A. pratensis and A. pubescens, 

 are perennial. 



Arrhenathertjm 



A. avenaceum. — Each spikelet contains two flowers ; 

 one male and one complete. They open simultaneously. 

 The stamens then elongate rapidly, so that in ten 

 minutes they have increased their length three- or 

 even four-fold, growing from 2-4 mm. in a minute. 

 They are at first stiff, but as they elongate they become 

 flexible, and when ripe they hang down, and the anthers 

 open at the point. The anthers curl outwards at the 

 tips, thus forming little cups, in which a certain quantity 

 of pollen accumulates. As long as the weather is calm, 



' Pringsheim's Jahrh. ix. (1873-4). 



