NOTES OX THE IDEXTIFICATIOX OF SOME BRITISH SPECIES 



OF CALLITRICHE 



By Herbert Jones 

 Department of Botany, University College of Wales, Aberystwyth 



Pearsall (1934) and Clapham (1952) point out the difficulty in identifying species 

 of Callitriche, with considerable justification. The main reason for difficulty is that 

 the only certain aid to identification is the characters of the mature fruit and often a 

 plant to be identified is in a sterile condition. Identification keys, therefore, are obliged 

 to contain \-egetative characters, which here mean chiefly leaf characters. The varia- 

 bilit\- of leaf form in some of the species, however, causes confusion. Again, it is some- 

 times stated that lower leaves of some species are linear, while upper leaves are oblong. 

 This does not take into account the mode of growth of the plant under different cir- 

 cumstances. It is perfectly possible, as will be shown, for lower leaves to be oblong 

 and upper leaves to be linear, a fact which would cause considerable confusion in 

 identification. 



A description follows of the general habit of growth in a species which displays 

 marked variation in leaf form. In view of the importance of fruit form in the identification 

 of species, a description of the fruit is also given. Four species are described : C. inter- 

 media HofEm., C. ohtusangula Hegelm., C. stagnalis Scop., and C. hermaphroditica L. (C. 

 autumnalis L.) 



Leaf form in C. INTERIvfEDIA 



When the crown is submerged, linear leaves only are formed. When the crown 

 arrives at the surface, obovate leaves are formed. Between the true obovate leaves and 

 the original linear leaves, however, transitional leaves will appear. Once the crown 

 arrives at the surface it will remain at the surface as long as the water level does not 

 rise. Elongation of the internodes must then result in older leaves being carried down 

 beneath the surface. It is evident then that beneath the surface will be found linear 

 leaves at the base of the shoot, then linear-obovate transitional leaves and finally obovate 

 leaves. In this way typically obovate leaves may be found well beneath the surface. It 

 is therefore misleading to refer obovate leaves to the floating state only. Again, if the 

 level of the water rises and the crown is submerged, then linear leaves will be formed. 

 In this case lower leaves may be obovate and upper leaves linear. It is therefore misleading 

 to refer to lower leaves as though they were necessarily linear, even though, from a con- 

 sideration of the development of the seedling, growing upwards from the bottom mud 

 of a pool, the sequence of events is certainly linear leaves at the base and then, when the 

 surface is reached, obovate leaves. 



Fruit Form 



The female flower of Callitriche consists of a syncarpous ovary, 4-celled by secondary 

 septation (Clapham, 1952), with typically 2 bracteoles. These latter may not be present 

 in some species. The 2 styles are terminal in the young fruit, but, due to developmental 

 changes with maturity, the styles may appear to arise from the sides of the ovary. When 



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