A Cytological Study of Pollen Development 
in Lactuca. 
BY 
R. RUGGLES GATES, PiI.D., F.L.S., 
Reader in Botany , University of London ( King's College ), 
AND 
E. M. REES, B.Sc., 
Demonstrator in Botany , Kings College. 
With Plates XVI-XIX. 
Introduction. 
I N the following account we propose to describe the cytological phe- 
nomena of meiosis in the pollen development of cultivated lettuce, 
Lactuca sativa , also the history of the tapetal cells and the later develop- 
ment of the pollen grains. Cultivated lettuce is generally supposed to have 
been derived from L. Scariola, L., and has been in cultivation for over two 
thousand years. But as L. Scariola occupies the whole of the Mediter- 
ranean region and Eastern Asia, it probably contains a number of micro- 
species, from one or more of which the cultivated varieties were originally 
derived. Some of the peculiarities of pollen development suggested a 
possible effect of this long period of cultivation. We have, therefore, com- 
pared the cultivated lettuce with the two wild species L . Scariola , L., and 
L. muralis , Fres., in all points in which an effect of cultivation on 
L. sativa seemed possible. 
The earlier stages of the work were completed, and a preliminary 
account of the results published by the senior author (Gates, 1920). All 
the later preparations and the whole of the drawings have been made by the 
junior author, and all stages of the development have been critically studied 
by us both. All of the cytological material was obtained from the John 
Innes Elorticultural Institution, Merton, through the kindness of the 
Director, Dr. W. Bateson, F.R.S., who is making a genetic study of certain 
cultivated lettuces and wild species of Lactuca . 
|Annals of Botany, Vol. XXXV. No. CXXXIX. July, 1921.] 
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