TAXONOMIC STUDIES IN THE GENUS CERASTIUM 



1. C. ATROVIRENS, C. PUMILUM, C. SEMIDECANDRUM 



By F. H. Whitehead 

 Department of Botany, Oxford University 



The treatment of ' critical groups ' is today the subject of much discussion amongst 

 systematists. Usually, these groups are those where the variation of morphological 

 characters between the groups overlap to such an extent that it is not possible to find 

 good correlation of these characters nor definite bimodality of the distribution of their 

 variation. Further confusion has been caused by the use of indefinite adjectives in 

 descriptions such as "long," "short," "large," "small," etc.; this leads to a very 

 subjective classification whose value it is difficult to test or assess. Appropriate biometric 

 and statistical methods can reduce the subjective element in classification, and some 

 of these together with their merits are described by Whitehead (1954). 



The annual species (section Viscosa) of the genus Cerastium are an example of 

 taxa which have given difficulty to taxonomists although the reason for such difficulty- 

 may not be immediately apparent. Of these species the following are the subject of the 

 present investigations : C. atrovirens Bab. (C. tetrandrum Curt.), C. semidecandrum L., 

 C. pumilum Curt., C. subtetrandrum (Lange) Murb. and C. glutinosum Fr. 



A close examination of these taxa shows that there is considerable variation and on 

 the basis of any one character complete separation is not possible. In this investigation 

 an attempt has been made to find characters relatively unaffected by growth conditions 

 and the following were chosen for use in the biometrical treatment which follows : 



1. Seed size - that is to say, the length of the seed multiplied by its breadth. Length 

 was taken as being the distance between the. funicle and the centre of the opposite arc. 

 Breadth was taken as being the greatest distance at right angles to the length. 



2. Pollen size. The pollen grains were found to be spherical if undamaged. The 

 dimension used was that of the diameter of the grain. 



3. Sepal/ Petal ratio. This was the average length of the sepals divided by the 

 average length of the petals. Sepal length was measured from the inner point of attach- 

 ment on the receptacle to the end of the scarious tip, if present, not including hair bases. 

 Petal length was measured from the inner point of attachment on the receptacle to the 

 extreme point of the longest of the two, three or four divisions of the petal. 



4. Bract/ Scarious tip ratio. This ratio was that between the total length of the 

 bract divided by the length of the scarious tip. The bract was taken as being the top- 

 most bracteole subtending a developed bud or flower. The length was measured from 

 the inner point of attachment on the stem to the extreme point, not including enlarged 

 hair bases. The scarious tip was measured from the point at which, in the centre of 

 the bract, the cells ceased to contain chlorophyll, to the extreme point of the bract, not 

 including hair bases. 



5. Petal /Cleft ratio. This ratio was that between the length of the petal and the 

 length of the cleft. Petal length was measured as in determining the sepal/petal ratio. 



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