574 
Hatfield, — A natomy of the Seedling and 
being - the ‘ girdles , of the mature structure. In the absence of internodes 
the courses of all these traces are nearly horizontal. 
The central meristematic dome of tissue soon becomes hollow, and 
the development of a starchy pith is distinguishable about half a 
millimetre behind the apex. In this region, cell-formation is restricted 
to a zone of tissue on the periphery of this central dome ; in this 
meristematic zone cell-division occurs by tangential walls only, and, as 
result of this method of growth, radial rows of starch-containing parenchy- 
matous cells are added to both pith and cortex (Text-fig. 6). 
Text-fig. 6. Portion of longitudinal section of stem, just below stem apex, showing the ‘ growth 
ring ’ or procambium, and cells of pith and cortex which have been formed by it. P. = pith ; 
G.R. = growth ring (procambium) ; c. = cortex ; l.t. = leaf-trace, details of which are omitted, 
x no. 
We have thus, in the primary tissues near the stem apex, a method 
of cell-formation which, in its restriction to a definite zone and in its cell- 
division in one plane only, resembles ‘ secondary ’ growth in thickness of the 
mature stem. This hollow cylinder — developed on the periphery of the 
central dome, the plerome, and ring-like in transverse section — probably 
represents the ‘ procambial ring ’ referred to by previous writers. 1 ’ 2 This 
method of growth adds very rapidly to the girth of the stem. A considerable 
amount of cortical tissue is interpolated between the central dome and the j 
tangential portions of the girdling leaf-traces, and thus these ‘ girdles 5 are j 
carried gradually, in the course of growth, to the periphery of the stem ; 
while maturing leaves are borne outwards rather than upwards away from 
the stem apex. 
1 Mettenius: Beitrage zur Anatomie der Cycadeen. Abhandl. Konigl. Sachs. Gesellschaft der 
Wissenschaften, v, 1861, p. 573. 
2 Robertson : Notes on the Anatomy of Macrozamia peleromera , loc. cit. 
