1918] CURRENT LITERATURE 75 
Abscission.—Hopcson” and KENDALL” have recently contributed to the 
literature on the abscission problem, the former having investigated foliar 
abscission in Citrus and the latter the abscission of flowers and fruits in 10 
genera of the Solanaceae, and particularly in Nicotiana. The investigation of 
an abscission problem may be expected to resolve itself into an effort to deter- 
mine the following points: (1) the histology of the tissue in which the abscis- 
sion takes place, and the position of the abscission zone therein; (2) the extent 
of the abscission zone, its histological differentiation, if any, and its develop- 
ment, that is, whether performed or not; (3) the position of the separation 
layer within the abscission zone, and the nature of the actual abscission pro- 
cess, that is, the method of cell separation; (4) the time of abscission, involving 
both reaction time and abscission time; and (5) the possibility of inducing 
abscission experimentally (by poisonous gases, mechanical injury, etc.). The 
most vital as well as, often, the most obscure of these matters which should 
receive consideration is the one involving the determination of the method of 
cell separation. In this connection both Hopcson and KEnpALt found, in the 
species investigated, that the abscission process conforms to the usual type 
which involves the separation of cells along the plane of the middle lamella. 
No cell divisions or elongations were observed to precede or accompany abscis- 
sion. Hopcson notes a remarkable swelling and gelatinization of the cell walls 
of the separation layer, which is followed by a dissolution of the gelatinized 
walls. In this case such cells, after functioning in abscission, resume growth 
and divide rapidly for a time. The abscission problem i in Citrus is of peculiar 
interest because of the well known shedding of immature oranges of the Wash- 
ington navel wiy which annually results in considerable financial loss to 
orange growers 
KENDALL’S es contains a more or less satisfactory consideration of all 
these points noted as of interest, but, as is perhaps inevitable in an attempt to 
cover so wide a field, no more than a beginning is made in working out some 
of the more fundamental problems. Thus, he shows that from water extracts 
of separation zones in which abscission has commenced a decidedly heavier 
precipitate comes down in 95 per cent alcohol than from those in which abscis- 
sion has not started. This difference is tentatively ascribed to the presence of 
pectin in the first case, it being derived from the hydrolysis of pectose during 
the dissolution of the primary cell membranes in the activated separation cells. 
This conclusion may or may not be justified, but such experiments indicate 
* Hopeson, R. W. , An account of the mode of foliar abscission in Citrus. Univ. 
Calif. Publ. Bot. 6: east 918. 
* KENDALL, J. N., eRe of flowers and fruits in the Solanaceae with special 
reference to pee Aa Ibid, 5:347-428. 1918 
, R. W., Some abnormal water relations in citrus trees of the arid 
= Hop 
Soiathnness: re their possible Rdvancs: Univ. Calif. Publ. Agr. Sci. 3:37-54- 
