202 



PHILOSOPHICAL NOTES ON 



show US how easily they might invaginate the stem if the leaf petiole 

 did not form such an obtuse angle with the stem. 



Nor is it Potamogeton alone that gives us indications of what 

 the vagina of grasses really means, for in the Polygonaceae we 

 have a vagina, called ocrea, enveloping the stem, independently of 

 the leaf. The vagina of Polypogon Monspeliensis (Fig. 64) 

 sufficiently shows us that in this species the vagina leaves the stem 

 and becomes adnate to the petiole, as in Potamogeton flabellatus 

 (Fig. 59), while in Potamogeton Rufescens (Fig. 55) we have 

 the homologue of the ochrea of Polygonum orientale. 



Fig. 64. Adnate stipule 



with ligule of Polypogon 



Monspeliensis, Desf. 



(Syme, pi. 1713.) 



Fig. 65. Glume and 

 awn of Alopecurns 



bnlbosus, Linn. 

 (Syme, pi. 1702.) 



If we follow the vagina of grasses further up the stem, we find 

 that it no longer envelopes the stem, but becomes the glume and 

 the pale of the spike; and the leaf or leaf -petiole becomes the 

 awn. The following Figs., 65, 66, and 69, remove all doubt about 

 this, for they are only reduced copies of the adnate stipule of 

 Potamogeton flabellatus (Fig. 59) and Polypogon Monspeliensis 

 (Fig. 64). 



If further evidence be needed of the identity of the glume and 

 pale of grasses with their leaves, we find it in teratological species, 

 when the ovules revert to ordinary buds. In Poa alpina (Fig. 68) 



