were stained by the safranin-fast-green technique. A 
projection apparatus was used in making tracings from 
comparable slides. 
Pulvini swellings from the axils of tassel branches of 
P39 maize were also sectioned, stained and projected in 
a manner similar to that used for the ears, because these 
small axillary protuberances appeared to represent 
another possible homologue of the prophyll. They were 
at maximum swelling when collected at the time of an- 
thesis. 
THe NATURE OF THE PHYTOMER 
Continuity of the phytomers. The phytomer, like the 
cell, was once considered to be the ‘‘true individual.”’ 
But now the plant as a whole is usually recognized as the 
individual, and the term ‘‘phytomer’’ is used to describe 
the level of organization represented by one repetition 
of its specialized regions or organs. The boundaries of the 
phytomer, and of the organs which compose it, are only 
approximate. Neither vascularization nor disarticulation 
delimit a discrete phytomer (Arber, 1984). Also such a 
unit is not necessarily delimited by the order of matura- 
tion, as in the classical segmentation of the phytomer 
used by Evans and Grover (1940) and others, because 
the degree and order of development of its various organs 
differ during vegetative and floral growth. 
In order to simplify comparison of its various mani- 
festations, we have chosen a phytomeric cycle comprising 
the group of organs which are adjacent to a given node 
or apparent node, as in the inflorescence where the nodes 
are usually obscure. This combination includes the leaf 
borne just below the node and its axillary bud with as- 
sociated prophyll just above the node, as well as the adja- 
cent internode (Plate I). The more classical delimitation 
of the phytomer at the nodes includes a leaf and bud 
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