318 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Silene acaulis and Gentiana acaulis, in the root of dandelion, and in 

 numerous rhizomes. Sometimes the pith is more developed, and the 

 vessels do not form a continuous ring. In Phyteuma hemisphericum 

 the pith is obsolete, but the medullary rays are very well developed, and 

 the vessels, which alone are lignified, form, in transverse section, narrow 

 radial lines accompanied by a parenchyma with thin cellulose walls. 

 Trifolium alpinum, Lotus corniculatus and others, show a structure 

 which may be regarded as intermediate between the preceding case and 

 a normal lignification of the secondary wood. The vessels are arranged 

 in radial threads, while the accompanying cellulose parenchyma includes 

 also supporting elements, consisting of elongated fibres of very small 

 diameter and considerably thickened walls, the middle lamella of which 

 shows the lignin reaction, while the internal thickening consists of 

 cellulose, showing sometimes a slight tendency to lignification. 



Reproductive. 



Morphology of Elodea canadensis.* — R. B. Wylie has studied the 

 morphology of this plant, which is one of the most specialised members 

 of the Helobiales. He describes in detail the development of the pistil- 

 late flower ; the long floral tube between the ovary and sepals is directed 

 towards the surface of the water by virtue of its low specific gravity 

 brought about by three rows of air spaces. In the male flower, which 

 is much simpler, the receptacle instead of pushing up into a floral tube, 

 becomes merely conical, and gives rise in turn to sepals, outer stamens,, 

 inner stamens, and very much later the corolla, which is not prominent 

 and may be quite rudimentary. Four megaspores are usually formed, 

 but in one instance six were observed ; the embryo-sac early develops 

 a pouch, in which the antipodal group of nuclei is formed ; the polar 

 nuclei approach one another at an early stage, and may remain for a 

 long time side by side ; their fusion was not observed before fertilisation. 

 The stamens produce two microsporangia each ; the pollen-grains adhere 

 in tetrads, and have a greater specific gravity than that of water. It 

 is of interest that the microspores, though borne by one of the most 

 specialised of submerged aquatics, entirely devoid of cutinised walls in 

 all its vegetative parts, have a strongly cutinised extine and a well- 

 developed intine. In Najas and Zannichellia on the contrary no extine 

 is developed. The extine in Elodea possesses spines which hold back 

 the surface film and imprison sufficient air to keep the spores afloat. 

 The microspore nucleus divides long before the grain has reached its 

 full size ; the generative cell is at first crescentic in outline ; after its 

 passage into the cytoplasm of the tube-cell it is for a time spherical, 

 but subsequently becomes much elongated, and just before its division 

 into the male cells is curved and may extend nearly across the spore. 

 The tube nucleus shows considerable irregularities in outline during its 

 existence. The formation of male cells occurs long before the pollen- 

 grains are shed ; they remain joined together by their elongated ends, 

 while in the pollen-grain they show marked cell-structure ; about the 

 nucleus is an extensive mass of cytoplasm differing considerably from 

 the contents of the spore and closely invested by a limiting membrane. 



"When the staminate flower is mature a bubble of oxygen forms at 



* Bot. Gazette, xxxvii. (1904) pp. 1-22 (4 pis.). 



