398 SCIENCE PROGRESS 



is a consequence of, and not a condition for, the increase in 

 size of the cells. This last is considered to result from decreased 

 elasticity of the cell wall. Subsequent growth renders per- 

 manent the change in cell size and shape. 



A number of investigations have been made recently on 

 leaf movements. Brenner (Svensk Bot. Tidssk., 10, 374-410, 

 191 6) has been able to show that leaves of Oxalis perform two 

 different kinds of movements, one due to change in light in- 

 tensity at dusk ("sleep" movements), and so-called autonomous 

 movements. These movements are influenced by temperature 

 as well as by light, but variations in humidity are without 

 influence. Leaf movements in Beech and Sycamore have been 

 examined by Lundegardh {Svensk Bot. Tidssk., 10, 438-70, 

 1 91 6). At the beginning of development the leaves are not 

 phototropic, and very little geotropic, but later in their de- 

 velopment the geotropism increases and this is found the con- 

 dition for the horizontal position. Two kinds of movements 

 have also been observed by F. C. Gates in leguminous leaflets 

 (The Plant World, 19, 42-5, 1916 ; Bot. Gdz. 61, 399-407, 191 6), 

 a movement being observed in continuous sunlight in addition 

 to the nyctitropic movement. The former the author terms 

 a " xerofotic " movement, on account of his attributing it to 

 differential water loss and consequent differential turgidity of 

 the two sides of a leaf exposed to insolation. The author was 

 able to induce the movement by application of local drying 

 agents. The movements of the leaves of a leguminous plant 

 (Phaseolus multiflorus) form the subject of a very suggestive 

 communication by Rose Stoppel (Zeitsch. f. Bot. 8, 609-84, 

 1 916), who was unable to find any relation between the move- 

 ments and a number of external conditions, such as temperature, 

 gravity, humidity, but she finds a correspondence between the 

 movements and electrical conditions in the atmosphere, and 

 therefore concludes that change in the atmospheric electrical 

 conditions is the stimulus bringing about the diurnal movement. 



A considerable amount of interesting work has been done 

 on irritability of lower plants. In regard to Algae Buder 

 (Jahrb. f. wiss. Bot. 58, 105-220, 191 7), working with a number 

 of unicellular organisms, has investigated phototactic move- 

 ments. Perhaps his most important conclusion is summed 

 up in the " law of the resultant," which states that with two 

 beams of light the resulting direction of action depends on the 



