222 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the tuber of Vxalis crassicaulis are compared with those in the stem- 

 tubers of the potato and artichoke, the root-tubers of tbe dahlia, and 

 other examples. Light was found to exercise a remarkably prejudicial 

 effect on the growth of tubers and rhizomes, whether belonging to the 

 root or to the stem. 



Biology of Helleborus fcetidus.* — Dr. F. Ludwig gives further 

 details of the mode in which this plant protects itself against the cold 

 in winter ; and classifies those plants which have special contrivances 

 for this purpose under three heads, viz. : — (1) Those whose aerial de- 

 velopment has already begun wheu the frost commences, — Jiemichimo- 

 nophilous ; (2) those with thin herbaceous leaves which persist through 

 the winter, — chimonochlorous ; and (3) those in which tbe chief develop- 

 ment takes place in the winter half-year, — cliimonophihus. Helleborus 

 fcetidus belongs to the third class. 



Influence of Moisture on Germination. f — Herr. W. Kinzel finds a 

 marked difference in different seeds in this respect. As a rule the 

 moisture of the soil has a greater influence in promoting germination 

 than the water absorbed by the seed itself. 



Resistance of Seeds to Mercury.+ — M. Casimir De Candolle states 

 that grains of wheat, after having been immersed in mercury for four 

 years, germinated, and produced normal plants. 



(3) Irritability. 



Chemotropism of the Pollen-tube.§ — Herr B. Lidforss has made 

 some interesting observations on the attractive force exerted by the 

 stigma on pollen-tubes, the species chiefly observed being Narcissus 

 Tazetta. He found that artificially prepared organic acids — formic, 

 acetic, lactic, succinic, malic, tartaric, citric — also amides, glucosides, 

 and tannins, had no undoubted influence on the direction of the growth 

 of pollen-tubes ; but that the almost immediate effect of introducing 

 into the medium a few grains of diastase was a deflection of all the 

 pollen-tubes towards the grains. Further experiment showed that the 

 constituent element of the diastase which attracted the pollen-tubes was 

 its proteid. Experiments on Narcissus confirmed the view that there 

 are chiefly two classes of substances which attract pollen-tubes, viz. 

 carbohydrates and proteids ; in other words, that the object of the move- 

 ment is simply the search for food-materials for their development. 

 Similar results were obtained with other species of Narcissus. The 

 pollen of most Liliaceae was found to be much more sensitive to mineral 

 salts than that of Narcissus. In tbe case of apopetalous Exogens, no 

 attractive force of proteids on the pollen-tubes could be detected ; but 

 in the case of a few Gamopetalse — Viburnum Lantana and nitidum — this 

 was observed. 



Localisation of the Sensitive Region in Geotropism. — From the 

 results of a series of experiments made on the germination of the seeds 



* Bot. Centralbl., Ixxx. (1899) pp. 401-13 (1 fig.). Cf. this Journal, 1898, p. 179. 

 t Landwirthseh. Versuchsstat., li. (1899). See Bot. Centralbl., Ixxx. (1S99) 

 p. 350. t Arch. Sci. Ph>s. et Nat., viii. (1899) pp. 517-8. 



§ Ber. Deutsoh. Bot. Gesell., xvii. (1899) pp. 23t;-4'2. 



