88 



BOTANICAL GAZETTE 



[JULY 



reproductive structures, because vegetative structures can be presented without 

 all sorts of details, and because reproductive structures lead so easily to general 

 questions and plant systems. Goeth's theory of metamorphosis is presented 

 and a modified view of this theory seems to dominate the discussion. This 

 general part occupies only 31 pages. 



The second, or special part, presents the various groups from the algae 

 up to the flowering plants, and occupies 120 pages. The reading should not 

 be very difficult for an educated man, interested in botany, but not a pro- 

 fessional botanist. The technical terms, especially those used in describing 

 flowering plants, are rather numerous. — Charles J. Chamberlain. 



NOTES FOR STUDENTS 



Recent work on tropisms. — For orthotropic organs Fitting and others 

 have proved that geo-excitation increases with the sine of the angle of displace- 

 ment from the normal or labile rest position. Therefore, the horizontal position 

 gives maximum stimulation, and all equal displacements below and above it 

 equal stimulation. Noack asserts that this physiological response does not 

 parallel any known change in the energy of gravity itself with variation of the 

 angle, and is not capable of direct physical explanation. 



Noack 2 inquires whether photo-excitation of orthotropic organs obeys the 

 sine of angle law. Assuming constant source of unilateral light, the intensity 

 falling on a given surface of the plant organ varies as the cosine of the angle of 

 deviation from the perpendicular, or as the sine of the angle of deviation from 

 the rest positions. On the basis of light intensity, therefore, one might expect 



the sine law to hold. 



finds, however, that intensitv of the illumination 



of the flank of the organ is not the sole factor, but the direction of the striking 

 rays is important, hence the sine law does not apply. He studied chiefly the 

 coleoptile of Avena. The following table shows the "Schwelle" (product of 

 light intensity in candle-power actually failing on the illuminated surface of 

 the organ by least time in seconds necessary to give a final response in 50 

 per cent of the organs) and presentation time under constant light source for 

 the various angles of deviation from the normal rest position. 





Angle 



Schwelle 



Presentation time 



if 



7-3 

 28.2 



30" 



9 5 

 19.0 



45" 

 11. 9 



16.8 





65° 

 11. 18 



12.9 



90- 

 12.2 



12.2 



I0 5 ° 



15.8 

 l6.4 



I20 



20.3 

 23.6 



135 

 23-7 



m O 



32 



33-SP4 



15° 



4 (C.M.S.) 

 8 



The "Schwelle" for the different angles shows the organ is more sensitive 

 to impinging light from above than from below the horizontal, and that this 



normal 



•gul 



Konrad 



zeption parallelotroper Organe. Zeitschr. Bot. 6:1-79. 1914. 



