50 SCIENCE PROGRESS 



virginiana, whilst swamp woods are dominated by Taxodium 

 distichum and Nyssa aquatica. 



The swamps themselves show a zonation parallel to 

 the water margin of which the zone characterised by Isoetes 

 Engelmani occurs at a depth of about 7 metres. This as 

 the water becomes shallower is followed in turn by zones domin- 

 ated respectively by Potamogetons Elodea canadensis, etc. ; 

 Nuphar and Nymphcea ; Scirpus lacustris, Typha, Phragmites, 

 Acorus, etc. ; a very mixed assemblage of amphibious herbs, 

 in which Hibiscus, Sium, etc., are characteristic. The floating 

 vegetation consists largely of Lemnaceae and Riccia natans. 



Fritch has carried out some interesting experiments on the 

 moisture relations of terrestrial algse {Ann. Bot., Dec), from 

 which it appears that they do not contain large vacuoles and, 

 even when air-dried, retain an appreciable proportion of moisture, 

 from 8-9 per cent., some of which probably represents retained 

 cell sap. 



The contraction on drying does not result in separation 

 of the protoplast from the cell wall to nearly the same extent 

 as in non-terrestrial species, so that when filaments are again 

 moistened, the water is readily absorbed by the living contents. 

 Compared with aquatic algse, the contraction of the terrestrial 

 alga appears to be much less. 



PLANT FHTSIOLOGY. By Prof. Walter Stiles, M.A.. Sc.D., 

 University College, Reading (Plant Physiology Committee) . 



Irritability and Movement in Plants. — ^A considerable mass of 

 data dealing with the reaction of plants to stimuli has 

 accumulated since this subject was last reviewed in these pages. 

 The contributions, as usual, come mostly, but by no means 

 exclusively, from the Continent of Europe, and practically all 

 the chief stimuli are included in the investigations. 



The stimulus of contact has been investigated by P. Stark 

 (" Das Resultantengesetz beim Haptotropism," Jahrb. f. wiss. 

 Bot., 58, 475-524, 1 9 19). Working with various varieties of 

 oat, and also with barley and Agrostemnta githago, he investigated 

 the effect of stimulating the stem by stroking it in more than 

 one place at the same horizontal level. If the stem is stimu- 

 lated in this way in two places and the intensity of stimulation 

 in the two places is equal, the stem takes up a position in a 

 plane bisecting the angle between the vertical planes containing 

 the two places at which the stroking is applied. As this angle 

 increases the response is less provided the intensity of the 

 stimulus remains the same. This result follows the parallelo- 

 gram law of the resultant of forces which had previously been 

 shown to hold in the case of phototaxis by Buder. The law 



