194 Alfred J. Eivart: 



times not till a year or two later. Crocker^ considers that this 

 is due to the greater impermeability of the seed coat of the later 

 seed to oxygen. It is thicker than that of the earlier seed, and 

 also mr.re impermeable to water, though not sufficiently so to 

 prevent its becoming fully saturated after a day's soaking. The 

 double effect can be overcome by a temperature over 30 deg. C, 

 which brings about partial anaerobic respiration, and by favour- 

 ing the absorption of oxygen hastens germination. The minimum 

 temperature for the germination of the naked seeds is 18 des:. C. 



Crocker has, however, overlooked the fact that both the early 

 and late seeda of Xanthium echinatum will germinate at 20 to 25 

 deg. C. if the temperature is maintained for 14 to 21 or more 

 days, instead of for 8 or 9 days. Hence the temperature 

 minimum for the germination of the later seeds is not as high 

 as Crocker states (32-34 deg. C). Further, if the burrs are 

 heated to 40 deg. C. for ai day, or to 50 deg. C. for a few^ hours 

 while soaking, a variable percentage of the later seeds will 

 germinate within 10 days. Hence the action of the intact 

 integument is merely to delay and not to prevent the germinai- 

 tion of the later seed at or near the minimum temperature for 

 the earlier of each pair of seeds in the burr. 



According to Crocker, Xanthium seed kept dry for 1 year, and 

 still more if kept in soil, germinates more slowly at 18-22 deg. C. 

 with the coats removed than do fresh burrs. This certainly does 

 not apply a-enerailly, since in very many old seeds there is no 

 perceptible dehiv in germination if the seeds are soaked and 

 kept at the right temperature. 



Light. — That in practice various small seeds must be super- 

 ficially sown to germinate well, and that this is directly or in- 

 directly due to their exposure to light is well known ; but 

 whether there are amy seeds which, whatever the other conditions 

 may be, will not germinate unless exposed to light, is uncertain." 

 Seeds of Vixrum album apparently only germinate when exposed 

 to light (Wiesner), and the same applies to the seeds of Tobacco, 

 according to Raciborski, and to those of Pan prate.nsis and 

 Apium yraveohns when fresh, according to Kin/el,'' whereas 



1 Bot. Gazette, li)06, p. 273. 



•2 For lit. sec I'feffer's Physiol., Kiiff. Ed., \a\. ii., p. S2. 



3 Kitizel, Her. (I. D. Hot. Ges., 1907, bd. xxv., p. 2(it». 



