480 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
its nutritive value for tlie organism attracted. The attraction will often 
be either positive or negative, according to the concentration of the 
fluid. The movements are not the result of either geotropism or helio- 
tropism. The actual perforation of the living cell-wall was seen, under 
the influence of an attractive fluid within the cell. 
Precisely the same phenomena were observed in the case of pollen- 
tubes ; they could in the same way be made to enter the stomates of 
leaves or to perforate cell-walls. For them sugar is an especially good 
irritant ; extract of meat, asparagin, peptone, and glycerin are neutral ; 
ammonium phosphate and sodium malate more or less repulsive. Not 
only the stigma, but a portion also of the style, the ovary, and the ovule, 
excrete a fluid which is attractive to pollen-tubes. Pollen-tubes are 
more or less hydrotropic, and often negatively aerotropic. The fluid 
excreted by one species is, as a rule, attractive also to the pollen-tubes 
of other species. 
Heliotropism.* — From experiments made chiefly on seedlings of 
Sinapis nigra , the late Mr. G. J. Romanes drew the conclusion that the 
heliotropic influence of electric sparks or flashes is greater than that of 
continuous illumination. Heliotropic curvature could be detected in 
vigorous seedlings within from 15 to 30 minutes, under the influence of 
bright sparks applied at as low a rate as 1 per minute. 
Heliotropic Sensitiveness-t — From experiments made on a variety 
of seedlings Dr. W. Figdor concludes that, as a general rule, shade- 
plants are more sensitive to heliotropic influences than sun-plants. 
With Lepidium sativum and others the lower limit of heliotropic sensi- 
tiveness was not reached at a distance of 7 m. from the source of light ; 
this is represented by less than 0*0003262 of a candle-power. 
Biology of Cuscuta. J — Mr. G. J. Peirce describes the phenomena 
connected with the parasitism and the twining of Cuscuta, the observa- 
tions having been made chiefly on C. Epilinum , europsea , and glomerata. 
Before the haustoria are formed by means of which the parasite receives 
food from its host, the supply of food takes place through certain 
papillate epidermal cells, to which the author gives the name pre- 
haustoria. The seedlings exercise a distinct selective power with respect 
to the objects round which they twine. The penetration into the host of 
the haustorial cells is accomplished by means of chemical action. 
The stems of Cuscuta have two distinct modes of twining. The first 
resembles that of ordinary climbers ; it is always in the direction in 
which they nutate, and is comparatively loose. At other stages, which 
alternate regularly with these, they make short, close, much more nearly 
horizontal turns about a vertical support, embracing it closely, and 
bringing their concave surfaces into intimate contact with it. The 
former mode of coiling is entirely the result of circumnutation and 
geotropism ; the latter of contact-irritation. Haustoria are ordinarily 
formed only upon the concave surfaces of the close coils, and are the 
result of irritation. 
* Proc. Roy. Soc., liv. (1894) pp. 333-5. 
t SB. K. Akad. Wiss. Wien, cii. (1893) pp. 45-59. 
X Ann. Bot., viii. (1894) pp. 53-118 (i pi. and 1 fig.). Cf. this Journal, ante, 
p. 82. 
