200 SUMMARY OF CURRENT RESEARCHES RELATING TO 



phenomenon of double fertilisation has at present been observed, L. 

 Guignard now describes it in the Solauacege (Nicotiana Tabacum, Datura 

 Isevis), and in the Gentiauaceae (Gentiana ciliata). These orders pre- 

 sent no important deviation from the process already described. With 

 regard to the formation of the endosperm, the difference in the time 

 when it takes place in these two orders indicates that there is not so 

 close a relationship as has been supposed by some between the septation 

 of the embryo-sac and its form and dimensions. 



In JNicotiana the oosphere is well distinguished from the two 

 synergids by its greater size. The two male gametes are formed only 

 during the passage of the pollen-tube down the conducting tissue of 

 the style ; they consist of little besides the nucleus. Their fusion with 

 the oosphere and the polar nuclei respectively takes place with great 

 rapidity. 



Hybridism and Xenia. * — C. Correns has studied very fully 

 hybridism and xenia in the various races of Zea Mays. The work was 

 begun in 1894 for the purpose of studying " the direct action of the 

 pollen on the fruit " (Xenia of Focke), and a preliminary account was 

 published in 1899. The author was then led to investigate the charac- 

 ters of the various hybrids produced. The investigation was carried 

 out on the lines of Gregor Mendel's well-known work on pea hybrids, 

 attention being paid only to certain selected characters in which the 

 crossed races differ from one another, and the behaviour of these charac- 

 ters investigated in relation to xenia and the hybrids. The observations 

 show clearly that on crossing there is produced a hybrid endosperm as 

 well as a hybrid embryo, a result which was at first very surprising, but 

 can now be clearly understood in the light of Nawaschin and Guignard's 

 discovery of double fertilisation. Correns discusses critically certain 

 other cases where the influence of the foreign pollen has been described 

 as extending to parts of the fruit other than the endosperm, and con- 

 cludes that the evidence for these is quite untrustworthy, and often 

 capable of another explanation. He affirms that xenia occurs only in 

 connection with the endosperm, as would be expected from the newly 

 discovered morphological features of fertilisation. By crossing races 

 which differ in certain pairs of characters (such as sugary and starchy 

 endosperm, colour of aleurone layer, &c), the exact behaviour of various 

 characters can be studied in the hybrid. He points out that the charac- 

 ters must be distinguished as independent and dependent, and in the 

 former category as free or conjugate. As Mendel showed, there are two 

 quite distinct stages to be studied in the behaviour in a hybrid of a pair 

 of characters derived each from a different race. The first stage is that 

 of vegetative development, in which there are obviously two extreme 

 cases: (1) one character only is developed, so that the hybrid shows 

 only the characters of one parent, the other being latent ; (2) both 

 characters develop, and the hybrid shows a new intermediate character. 

 Pairs of characters which behave in the first manner Correns calls hetero- 

 dynamic, one being dominant and the other recessive, in Mendel's 

 terminology. Those of the second class he calls homodynamic. The 

 other stage to be studied is that of the relation of the characters to ti.c 



* Bibliotheca Botanica, Heft 53, 1901, 161 pp. and 2 pis. 



