90 R. A. BRINK 



forms in the absence of pollination, as may be demonstrated easily by re- 

 moving the corollas and anthers — by cutting off the distal portion of the 

 head in the bud stage. Ordinarily the anthers do not open in the intact 

 mature flower. 



The female gametophyte is formed without reduction in chromosome 

 number of the nuclei. Otherwise it is a typical eight-nucleate, seven-celled 

 structure lying in direct contact in the mature ovule with the innermost 

 layer of cells of the single thick integument. The polar nuclei fuse to give a 

 hexaploid primary endosperm nucleus. The single layer of cells comprising 

 the nucellus disintegrates during formation of the embryo sac. 



Sexual forms of the common dandelion are not known to occur. Accord- 

 ingly another species, T. kok-saghyz, the Russian dandelion, was examined 

 as a control. T. kok-saghyz is diploid (2x = 16) and, since it is self-incom- 

 patible, requires cross-pollination for seed formation. A comparative study 

 of T. officinale and T. kok-saghyz was made with a view to discovering, if 

 possible, the means by which the former is enabled to dispense with the 

 secondary fertilization, which is essential to seed formation in the latter. 

 Heads were collected at four stages: late bud, just prior to anthesis, open 

 flower, and with seeds ranging up to six days of age. After sectioning and 

 staining, the number of cells in the endosperm and embryo was determined, 

 and observations were made on the amount and distribution of food ma- 

 terials. 



Seed formation in T. kok-saghyz follows the course typical of the angio- 

 sperms. Endosperm and embryo development are initiated by double 

 fertilization. Subsequently, the two tissues grow very rapidly, and in tune 

 with each other. Cell number in the endosperm increases exponentially. The 

 endosperm, however, is somewhat less precocious than in most flowering 

 plants. The seed is mature 9-12 days after fertilization. 



A markedly different set of relations present themselves in the seed of 

 the apomictic T. officinale. The seed in this species begins development when 

 the flowers are in the late bud stage. By the time the flowers open, there may 

 be 100 cells or more in the endosperm, the embryo, or in both tissues in some 

 seeds. A further significant fact is the extraordinary amount of variability 

 in the size ratios of endosperm and embryo from seed to seed of even age. 

 There is a positive relation between cell number in endosperm and embryo 

 over the period studied — as would be expected in view of the fact that in 

 most seeds both tissues are growing. As measured by the correlation co- 

 efficient, this value is low (r = .57) compared with that for T. kok-saghyz 

 (r = .76). 



Average cell number in the embryo in relation to endosperm size is de- 

 picted for the two species in Figure 5.3. Cell number in the endosperm in- 

 creases geometrically, so that size of the tissue may be expressed appropriate- 

 ly in terms of division cycles. Embryo cell number, in contrast, increases 



