204 



6. Elodea. The common Waterthyme, Elodea canadensis is well 

 known for having trimerous whorls of leaves; one would therefore 

 think it out of place to treat this plant hère. 



There are however in Elodea sometimes slight déviations from 

 the normal whorled condition, just in the same way as those des- 

 cribed in Polygonatuni. 



Eichler describes in his „Blûthendiagramme"^ that the flower 

 arises in the axil of a leaf of a tetramerous whorl, which is f ollowed 

 on the main axis by a dimerous one; the mutual position of the 

 leaves is so (cf. fig. 40 B), that the lower tetramerous whorl consists 

 of a trimerous one with one supernumerary leaf, taken from the 

 next higher whorl, of which remain only two leaves. 



In not-flowering végétative axes the same phenomenon will 

 sometimes occur without any flower or axillary shoot; a growth 

 whorl of 3+ 1 leaf is f ollowed by another of 2 leaves. 



More clearly the some phenomenon is showed by E. densa 

 Casp. of which I studied some shoots out of our botanical garden^. 

 Hère the axillary buds never occur in the axil of a leaf of a normal 

 whorl, but where a bud is présent the whorl in which it occurs 

 is always united to the whorl below. In trimerous shoots the whorl 

 gets in this way six members, in tetramerous shoots eight. In 

 numerous cases slight irregularities are added; mstead of a double 

 whorl of six leaves we find e. g. one of 3 + 2 leaves, while the next 

 one counts 1+3; or instead of three tetramerous whorls there 

 are developed two of 4+3 and 1+4 leaves. In thèse cases the 

 original alternation of the whorls is always retained ; the first grovyrth 

 whorl consists of one regular tetramerous whorl and three leaves 

 placed clearly over three of the four spaces; the next growth 

 whorl then contains one lower member, falling over the fourth 

 space, and an alternating regular tetramerous whorl. 



On a much smaller scale we hâve therefore just the same pheno- 

 menon as in Polygonatum; we may say that Elodea has ordinary 

 whorled leaves, with a slight tendency to thef ormation of growth whorls. 



1 Eichler. 1. c, I p. 92. 



2 No. 11057 of the Catalogue. 



