LEAVES WITH EEGAED TO LIGHT. 429 



verse plane are written with a + sign, those behind it with a — 

 sign. When the plant is growing the right way up, not in 

 the klinostat, I have given the angles as above and below the 

 horizon, which in this case of course coincides with the transverse 



plane. 



Dialeliotropic plane. — The expression diaheliotropism has been 

 used by my father (pp. cit. p. 438) as a more convenient term 

 than any equivalent of the German " Transversal-Heliotropismus," 

 to mean the power possessed by leaves of placing themselves " more 

 or less transversely to the light." In accordance with this termi- 

 nology, I shall call the plane at right angles to the incident light 

 the dialieliotropic plane ; and leaves in this plane may be said to 

 be in the dialieliotropic position. 



Upper and lower— These terms applied to leaves mean, of 

 course, the morphologically upper aud lower sides. 



Normal and inverse positions.— -When the plant is not on the 

 klinostat, but growing naturally, it is said to be in the normal 

 position when the axis of the plant is vertical and the stem above 

 the root. If the plant is upside down with its axis vertical, it is 

 said to be in the inverse position. 



Experiments ivith Eanunculus Ficaria. 

 These plants were especially well adapted for my experiments, 

 being excessively common, and obtainable at an early period of 

 the year, when many plants refuse to flourish properly. More- 

 over they grow healthily indoors under the conditions to which it 

 was necessary to subject them. The method which I adopted 

 consisted in wrapping the roots in damp cotton-wool, and cover- 

 ing this with an envelope of sheet india-rubber. Under these 

 conditions the plants remained perfectly healthy, and put out 

 numerous young leaves. The roots of the plant could then be 

 tied on to a large pin ; and this could then be fixed in various posi- 

 tions into a cork disk attached to the klinostat; and thus the 

 plant could be illuminated from above, or below, or latera ly, as 

 might be desired. In other experiments, not made on the klino- 

 stat, the plants were fixed to the cork lining of the cover of a 

 jar which was partly filled with water. They were thus culti- 

 vated in damp air, in the manner employed by Sachs in his 

 experiments on bean-roots. In other cases the plants were grown 

 with their roots in vessels of water. Lastly, other plants were 

 simply grown in flower-pots. 



