444 University of California Publications in Botany [Vol. 5 



of plants to illuminating gas. Flower-fall was not reported upon, but 

 the response so far as leaf-fall is concerned received attention. A 

 large number of species was placed in atmospheres containing various 

 concentrations of illuminating gas and some of its constituents. A 

 striking relation was found to exist between the concentration of the 

 gas and the extent of leaf-fall. Thus, to take a few examples, in Salvia 

 splendens 1000 p.p.m caused the oldest leaves at the base of the stem 

 to fall, while 5 p.p.m. caused less shedding; in Datura stramonium 

 4000 p.p.m. caused the fall of all leaves except the youngest, whereas 

 with 5000 p.p.m. only the oldest leaves fell, and in Hibiscus rosa- 

 sinensis the same results were obtained when 4000 and 1000 p.p.m. 

 were used. In this connection it is interesting to note that in the case 

 of Citrus the opposite relation was foiuid to hold, namely, that the 

 youngest leaves were the first to be shed. Thus, in the case of a potted 

 seedling subjected to an atmo.sphere containing illuminating gas, 

 shedding began with the terminal leaves at thirty hours and gradually 

 extended down the stem until all the leaves had fallen at fifty-four 

 hours. 



The distinction between flower and leaf abscission is emphasized 

 in the relation, pointed out by IMiss Doubt, between the age of the leaf 

 and the readiness with which abscission is induced. She found that 

 the older the plant, the less gas was required to cause the older leaves 

 to fall; the younger leaves in all cases being least affected (cf. Lloyd, 

 1914, p. 69 and 1916, p. 57). This situation might be explained by 

 the fact that in the case of the decidedly deciduous species the period 

 during which the older leaves would normally be attached to the 

 plant was less than that of the younger leaves and that thus the 

 abscission layer was already partially formed or more readily activated. 

 In the absence of controls further speculation on this point is useless. 

 In contrast to these responses of younger and older leaves, we have the 

 following situation in the case of floral abscission, at least so far as 

 Nicotiana is concerned. Here the period preceding the partial matur- 

 ing of the fruit is, almost without exception, the only one in which 

 flower-fall can take place, since the rapid development of mechanical 

 tissue makes fall nearly impossible thereafter, even if a separation 

 layer is found among the cortical cells which remain undifferentiated. 

 Thus buds, young flowers, and partially matured seed capsules alone 

 are shed — and the latter only rarely. To take one example, it is a 

 common experience in the field to find that as a result of low tempera- 

 ture there is a progressive fall of the smallest buds and then larger 



