D. M. BARLING 



211 



sides of the stigma. The stigma opens before it has fully emerged above the anthers and 

 pollen readily finds its way on to it. This introrse dehiscence has been found on marked 

 capitula before the involucre has opened, and thus a cleistogamous behaviour may be 

 common in the outer and older florets of any head. 



After fertilisation, ovary development is rapid, so that the tips of the outer florets 

 may be lifted above the involucre even though the central florets may be undeveloped. 

 These fertilised florets soon show a browning with orange-tinging of the corolla and 

 browning of anthers and stigma. When all florets are fertilised, the capitulum remains 

 closed; thus closed and open capitula are to be found on the same plant. As the ovaries 

 increase in length, the withering floral parts are eventually ejected past the tips of the 

 involucre and the pappus becomes visible. When ripe, the achene-bearing capitula open 

 during fine weather and do so well into the evening. The development of capitula is 

 uneven and so all stages from flowering to ripe achenes may be found on single plants. 



The effect of ovary elongation is important in relationship to the identification of 

 the subspecies and records should only be made in the morning when flowering capitula 

 will be open and fertilised heads closed. Partially fertilised heads show florets of variable 

 length and only the true flowering heads should be used for classification. 



The extreme unevenness of emergence and ripening of capitula prevented assessment 

 of the number of achenes produced by individual plants. Thus mature heads were 

 taken at random during the season and the mean number of achenes per capitulum 

 calculated. The mean number of fruiting heads per plant was then calculated in late 

 August and the multiple of these gave an approximate output of 589 achenes per plant. 



The seed gave an 85 per cent germination so that the effective reproductive capacity 

 was 480-65 per plant. The achene weight was calculated by weighing three samples 

 of 100 achenes from which the long beaks had been removed. The achene weight was 

 0-00744 grams. 



Basal branching from the lowest axillary buds is a characteristic feature of the 

 species and a mean number of 5-6 ± 0-62 main branches per plant was obtained by 

 late August sampling. 



The number of ligules per capitulum was 8-39 ±0-11 and indicates a variability 

 which is considerable even within single plants. 



The number of florets per capitulum was 55-35 it 1-07 and it is interesting to 

 compare this with the number of achenes per capitulum. It is seen that there is con- 

 siderable difference, probably due to competition in the capitulum, since empty achenes 

 are invariably in the centre of the head. The latter cannot be taken as a means of 

 calculating fertility. 



T. pratensis has a chromosome number of 2n = 12 (Winge, 1938), andOwnbey (1950) 

 has given the same number for North American material morphologically identical 

 with subsp. pratensis. Clapham (1952) gives two chromosome numbers, 2n = 12 for 

 subsp. minor and 2n = 14 for subsp. pratensis. In the present work, meiosis in anthers 

 has been studied in detail and observations on root tips and other somatic tissue, such 

 as the corolla, have been made. In all approximately 200 plants have been counted by 

 the Feulgen squash and aceto-carmine squash techniques and permanent preparations 

 have been made (Darlington & La Cour, 1947). The plants were selected from a wide 

 variety of habitats and distribution and in all cases 2n 12. P.M.C. meiosis has regu- 

 larly shown six bivalents. At diakinesis the shorter chromosomes normally form ringlike 

 structures having two chiasmata, usually terminal in position. The one long bivalent 

 usually shows three chiasmata. One feature, common at anaphase and telophase, is the 

 lagging in the separation of the long bivalent so that a chain-like appearance results. This 

 is probably the same as the bridge-like structure described from one plant by Ownbey 



