176 SUMMARY OF CURRENT RESEARCHES RELATING TO 



being concrescent in their lower part. A similar multiplication of the 

 pollen-sacs also occurs, though not so often. When the number of 

 pollen-sacs is three or four, they form a true sorus, comparable to that 

 of the Cycadese. 



The author points out the importance of these facts as indicating 

 that the Cycadeaj are not the most ancient Gymnosperms, from which 

 the Coniferae are descended, since the latter present a still more ancient 

 form in Salisburia, from which the Cycadeae are derived. The Ging- 

 koaceaa, Taxaceae, and Pinaceee must bo regarded as three distinct groups 

 of Coniferae. 



Distribution of the Sexes in Dioecious Plants.* — Prof. S. Stras- 

 burger has made a long series of experiments on growing Melandrium 

 album (Lychnis vespertina) under different conditions, with a view of 

 determining the influence of external conditions in promoting the pro- 

 duction of male or of female plants. The conclusion arrived at is that 

 no material effect is produced by external conditions, the relative pro- 

 portion of the sexes being, within certain limits, fixed by heredity. The 

 only mode of varying the relation appears to be by artificial selection. 



A number of observations are also recorded on the well-known 

 phenomenon of the so-called castration of the male flowers of this plant 

 by the attacks of the parasitic fungus Ustilago violacea. This parasite 

 stimulates the development, in the female flowers, of the rudiments of 

 stamens up to the point when the pollen-mother-cells are produced in a 

 normal manner. But the contents of these cells are then consumed by 

 the parasite, the anther-lobes becoming entirely filled up by its violet- 

 coloured spores. The development of the pistil is also arrested before 

 it arrives at a stage capable of impregnation, and other characters of the 

 male form are assumed. The individuals which present these j)heno- 

 mena are, however, not male plants, as is often stated, but female 

 plants. 



Cleistogamous Flowers. f — W. Eossler has studied the cleistogamous 

 flowers of Juncus bufonius and Oxalis AceloselJa. 



In Juncus bufonius all the grains of a pollen-tetrad may put out tubes. 

 The pollen-tubes do not take the shortest way to escape from the anther, 

 but display many windings. They always emerge from the lateral 

 furrows between the pollen-sacs. After leaving the anther, the pollen- 

 tubes do not always find their way to the stigma. Those that reach the 

 ovary always enter the ovules through the micropyle. 



In Oxalis Acetosella there is a difference in the period of maturity of 

 the epipetalous and hypogynous stamens ; the former put out their 

 pollen-tubes somewhat earlier. The pollen-tubes do not always escape 

 through the sutures of the anthers ; they sometimes pass through the 

 anther-wall. As in the previous case, the pollen-tubes do not all find 

 their way to the stigma. Intermediate forms occur between the cleisto- 

 gamous and the chasmogamous flowers. 



Influence of the Number of Pollen-grains on Fertilisation. % — 

 C. Correns has made a series of observations — chiefly on Mirabilis 



* Biol. Centralbl., xx. (1900) pp. 657-65, 689-98, 721-31, 753-86. 

 t Flora, Ixxxvii. (1900) pp. 479-99 (2 pis. and 1 fifr.). Cf. this Journal, ante, 

 p. 58. J Ber. Deutsch. Bot. Ges., xviii. (1900) pp. 422-35. 



