ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 57 



the oospheres are formed without reduction of the chromosomes, and 

 develop into embryos without being fertih'sed. In H. exceJIen-s and 

 H. flageUare the embryo-sac formation somewhat resembles tliat of 

 Thalictrimi pur-pur ascens, but in most seeds apospory also occurs. The 

 embryo-sac formations in these plants is noteworthy as being different 

 from those of any other previously described parthenogenetic plants. 



Post-floral Growth of Sepals in Convolvulaceae.* — N. SvedeHus 

 has studied the calyx in a few Convolvulaceae with special i-eference 

 to its development into a water-cup. In this connection he has also 

 examined the epidermal hairs which in this family often assume 

 secretory functions. Secretion, which is most marked in the fruiting 

 stage, almost always accompanies a more or less vigorous post-floral 

 growth of the sepals, but in no type is it so great as in Stktomrdia, 

 where the fruit ripens in a water-bath, surrounded by the greatly 

 enlarged sepals. Here the secretion is of a watery nature, but in 

 Operculina and Ipomma alata and /. tnberosa mucus is secreted. 



The function of the post-floral calyx and the accompanying secretion 

 is to form a protection against the drying which would result from such 

 strong insolation as that to whicli the liane-like Convolvulaceae, e.g. 

 Stictocardia, are exposed. Where the sepals are strongly developed the 

 fruit-walls are thin and soft, e.g. various species of Dillenia. There is 

 much variation in the post-floral development of the sepals, which may 

 be simply folded together, or may be entirely or partly thickened. In 

 Porana and in some species of Ipomma, the sepals form wings for the 

 fruit, while in Cardiochlamys they form a bladder-like covering. 



Where secretion occurs, all the sepals may be involved in it, as in 

 Stictocardia, or it may be confined to the inner ones as in Operculina, 

 where the three inner sepals are secretory and the outer ones are 

 protective. 



Clandular hairs similar to those on the sepals may also occur on the 

 foliage-leaves, being confined to the lower surface. The hairs on the 

 outer side of the protective sepals are like those on the upper surface of 

 the foliage-leaves, and are probably hydathodes. 



Sexual Differentiation. t — A. F. Blakeslee has investigated the 

 differentiation of sex in thallus gametophyte and sporophyte, especially 

 among the Mucorineae. He finds that the " homothallic " forms are 

 anisogamous, but that the " heterothallic " forms are isogamous. It is 

 probable that isogamous " homothallic " forms have given 'rise to two 

 lines of descent, viz. anisogamic, homothallic forms, and isogamous, 

 " heterothallic " forms. At present no theory can be formulated as to 

 origin of sexuality in the group. Where there is difference in vegetative 

 growth, the + strain is more luxuriant, but the reason for this is not 

 evident, since the zygote is suspended midway between the two thalli. 

 The sexes are mutually attractive, but it is not yet proved that the 

 terms male and female may be substituted for + and — . The author 

 then draws a comparison, from the sexual point of view, between the 

 Mucorineae, Bryophyta, Pteridophyta. and Phanerogams, and arrives at 



* Flora, xcvi. (1906) pp. 231-59 (31 figs.). 



t Bot. Gazette, xlii. (1906) pp. 161-78 (1 pi. and 3 figs.). 



