392 BRITISH FLOWERING PLANTS chap. 



by various workers in plant physiology.^ Kerner recorda 

 that when the outside air was 15° C. the interior of the 

 spathe rose to 40° or even 44°. 



AcoBTJS (Sweet Flag) 



Protogynous, complete flowers forming a spike. 

 Perianth consisting of 6 short scales, with 6 stamens. 



A. Calamus. — -The spike contains many hundred 

 flowers. It never, however, produces ripe fruit in 

 Europe, though it does so in Asia. It is probable that 

 this is due to the absence of the proper insects for 

 fertilisation. Ludwig, on the other hand, accounts for 

 it by suggesting that all our European plants are 

 descended from a specimen brought from the east by 

 Clusius. Bentham, however, regards it as native in 

 some of our eastern counties. 



LEMNACE^ 2 



Small floating plants of remarkably simple structure, 

 consisting of repeatedly branching so-called " fronds," 

 which show no distinction of stem and leaf. In Wolfiia 

 there are no roots, or vascular tissue, the plant consist- 

 ing of little green cellular cushions. The flowers, which 

 are very rare, appear in a fissure at the edge of the 

 shoot. They are reduced to a small spathe-like bract 

 enclosing one or two stamens, and a pistil (Fig. 320). 



Lemna (Duckweed) 



We have four species. L. polyrhiza has the roots 

 in clusters. In the other three they are single. L. 

 trisulca has the fronds very thin, oblong, or narrowed 

 at one end. In the other two they, are broadly ovate, 

 in L. minor nearly flat, in L. gibha very thick and 

 convex underneath. L. trisulca is submerged, the 

 others float on the surface of the water. 



^ Vines, Physinlogy of Plants, p. 304. 

 ^ Hegelmaier, Die Lemnaceen. Leipzig, 1868. 



