PLANT PHYSIOLOGY 51 



also holds when the stem is stimulated in three or four different 

 places at once. 



That twining plants were sensitive to contact, in some cases 

 at least, had been shown by Loffler as long ago as 191 3. The 

 same author has now extended his researches to a large number 

 of species belonging to different genera (" Experimentelle 

 Untersuchungen iiber Regeneration des Gipfels und Kontak- 

 tempfindlichkeit bei Windepflanzen," Ber. deut. hot. Ges., 37, 

 6-24, 191 9). 



Reaction to the gravitational stimulus has been a favourite 

 subject of investigation. Miss T. L. Prankerd, a firm upholder 

 of the statolith hypothesis, has discovered a tissue in the haulm 

 of wheat composed of cells containing a single crystal of calcium 

 oxalate. ('< Statocytes of the Wheat Haulm," Bot. Gaz., 70, 

 148-152, 1920.) This, it is thought, may be a statolith for 

 geo-perception, the organ being very sensitive to gravity when 

 the haulm is laid horizontal. The rate of fall of the crystals 

 when the tissue is inverted is lOyu, a minute, a much higher 

 rate than that found for falling starch grains, a fact which may 

 be correlated with the sensitivity of the organ. 



The same author has recently published an account of the 

 irritabihty of fern fronds, reaction to gravity being specially 

 investigated (" On the Irritability of the Fronds of Asplenium 

 bulbiferum, with Special Reference to Graviperception," Proc. 

 Roy. Soc, B, 93, 143-152, 1922). It is shown that the life- 

 history of the frond of Asplenium bulbiferum can be divided 

 into three periods characterised not only by differences in 

 external morphology, but also by differences in response to 

 stimulation and in cytology. In the first stage, while the 

 frond is still curled, the sensitivity to gravity and hght is less 

 than during the middle period, when the frond is uncurling and 

 when the irritability both to light and gravity is at a maximum. 

 In the third stage, when the frond has uncurled, geotropic 

 irritabihty has stopped and irritabihty to light soon does so. 

 The development of statocyte tissue runs parallel with the 

 development of geotropic irritabihty. In the first stage the 

 statocyte tissue is increasing to a maximum which is reached 

 in the middle period, while by the time the frond has uncurled 

 the statocytes have disappeared. 



The relation between the presence of starch grains in the 

 endodermis of members of the Compositas and their ability to 

 react to gravity has been examined by Clara Zolhkofer, parti- 

 cularly in the case of Tagetes. (" Uber das geotropische Ver- 

 halten entstarkter Keimpflanzen und den Abbau der Starke 

 inGramineen-Koleoptilen,"5er. deut. bot. Ges., 36, 30-38, 1918.) 

 If young seedlings up to four days old are placed in the dark 

 for three or four days the starch in the hypocotyls disappears. 



