188 



NOTES ON SOME BRITISH SPECIES OF CALLITRICHE 



a few days for details of venation to be clearly seen. For subsequent examination, leaves 

 were transferred through a range of alcohols to water and mounted between glass in 

 the negative carrier of a photographic enlarger. Photographic negatives were then made 

 by projecting the image of the leaf on to a lantern plate, a millimetre grid being also 

 projected on to the same plate to give the scale. The projected image from these negatives 

 was then traced at suitable magnifications. 



Aid in identification of the species was derived from : Hegelmaier (1864), Samuelsson 

 (1925), Pearsall (1934), Butcher and Strudwick (1946) and Clapham (1952). Nomen- 

 clature follows that of the last quoted.* I should like also to acknowledge gratefully the 

 facilities afforded me to ex?imine the Callitriche species in the Welsh National Herbarium 

 at the National Museum of Wales, Cardiff. 



Since this account was written, a paper has appeared on the section Eucallitriche 

 Hegelm. in the Netherlands, containing an account of the distribution and morphology 

 (including chromosome morphology) (Schotsman 1954). In general (where the same 

 morphological features are referred to) there is close agreement between the description 

 of the first three of four species which follows and Schotsman' s description of these species, 

 if allowance is made for Schotsman's retention of the name C. hamulata Koch (with cogent 

 reasons given for this) while in the present paper C. intermedia G. F. Hoffm. is used. 



In the following description, where comparative statements are made, these refer 

 only to the four species of Callitriche described. 



A. Callitriche intermedia 

 Leaf form (Fig. lA 1-5) 



The greatest range in leaf form is shown here. The extreme linear leaf is up to 



4 cm. long by 1 mm. broad and has a pincer-like tip. While an emarginate tip may 

 be seen on leaves in still water, the exaggerated pincer tip is displayed by leaves of plants 

 found in running water. This may be connected with the maintenance of shoots in a 

 horizontal or inclined plane by the water current since shoots submerged in still water 

 in a horizontal plane produce leaves with pincer tips. The extreme obovate leaf is up 

 to 2 cm. long by some 5 mm. broad. The tip of the leaf is emarginate, as indeed are all 

 leaves of all four species described. The outline of the obovate leaf is of little use in 

 separating C. intermedia from C. ohtusangula. Figs. lA 3 and 5 show how the tip of the 

 leaf may be rounded or more or less pointed. The leaves 3, 4 and 5 are all from the 

 same shoot, being separated from each other by 3 nodes. Leaf 3 was formed when the 

 crown was at the surface. The level of the water then rose and leaf 4 is the result of 

 submergence on the development of a leaf which had attained a certain stage under 

 aerial conditions. The influence of submergence is greater on the basal part of the leaf 

 since in C. intermedia this part develops last. Accordingly this leaf has a rounded tip 

 and is attenuated basally. The crown subsequently arrived at the surface once more. Leaf 



5 shows a somewhat pointed tip while basally it is broader than leaf 3. Leaf 5 reflects 

 the influence of submergence in early development through its pointed tip while the 

 later conditions of floating existence have resulted in the broader base. While, in the 

 other 3 species considered, it appears that the range of leaf form is less marked, at the 

 same time the writer considers that leaf shape cannot be relied upon in identification, 

 unless the history of the shoot with regard to submergence or emergence is known. 



In the obovate leaf 3 veins are present with possibly branching from the 2 lateral 

 veins. In the linear leaf a single vein occurs. It appears that leaf shape and venation 

 are controlled by separate factors. In the transition from obovate leaf to linear, it is 



• Except for C. autumnalis which, according to Mr. J. E. Dandy, is an illegitimate substitute for the earlier C. hermaphroditica L. 



