Further Studies on tlie Apogamy and Hybridization of the Hieracia. 245 



off or is removed. Notwithstanding tl.is violence, tlie heads of the 

 apogamic species develop themselves undisturbedly and the fruits 

 ripen as usual, but the heads are, in the ripe state, when they have 

 opened, easily distinguished from the intact heads by the short pappus, 

 which renders them much smaller in size and less conspicuous. 



By this "castrating" process we would imagine that every possi- 

 bility of fertilization has been removed. The objections which might 

 be raised, can all be refuted, and they fall to the ground before the 

 cytological investigations of ROSENBERG (1906, 1907) in the apogamic 

 species. The seeds of the castrated head are always sown in order 

 to test their germinating power, and the plants raised are generally 

 preserved till full development to compare them with their parents. 



In the Hieracia — as in the other Compositae — not all the fruits 

 in a head develop, so that they contain a seed capable of germi- 

 nating. With a little practice one can rather easily distinguish with 

 great probability the full fruits (that is, those which have seeds 

 capable of germination) from the empty ones, merely from their outer 

 appearance. The empty fruits are generally somewhat smaller and 

 more slender and of a paler colour (pale-brown to dark-brown), whereas 

 the full ones are bigger and plumper and, in most species, of a black 

 or black-brown colour (in some species brown in different shades, in 

 a few species pale-brown, almost straw-coloured). The percentage of 

 empty fruits in a head varies greatly in the different species, in the 

 different plants of the same species, and even in the same plant, 

 according to the season and from year to year, but the heads which 

 ripen about the same time in the same plant have no doubt about 

 the same proportion of empty fruits and full ones. 



Now it may be supposed that this proportion is altered by the 

 castration. This seems probable considering that certain species of the 

 subgenus Pilosella, which produce fruits apogamically, are also able to 

 hybridize, which requires a fertilization of those ovules that develop 

 into seeds from which hybrids come forth. The investigations of 

 Rosenberg have also shown that in these species a few normally 

 developed embryo-sacs are found. In such cases the number of full 

 fruits should be smaller in castrated heads than in heads which have 

 had an opportunity for pollination from the visits of insects, so numerous 

 in these plants (especially of bees, humble-bees and butterflies). 



By examining a sufficient number of castrated and non-castrated 

 heads of the same plant one should now be able to decide the question. 

 This means, in other words, to answer the ciuestion, whether the 



