10 



L. HILLIS-COLINVAUX 



(Section III). Only in the species cuneata is the uncorticated region 

 more extensive, and this is described in the discussion of this species. 



In the spaces of the cortex and medulla (Figs 6, 7), outside the 

 filaments, the deposition of aragonite occurs (Askenasy, 1888; Wilbur 

 et al., 1969; Borowitkza and Larkum, 1976a, b). Calcification, there- 

 fore, takes place entirely outside the filament. 



Fig. 8. H. monile, surface view of peripheral utricles of a segment about 48 hours old. 

 The ends of the filament, or peripheral utricles (large circles or polygons) adhere 

 to form a continuous outer surface. The corners are less rounded in older segments. 

 Scale bar is 10 [im. 



B. Wall structure in a cellulose-free plant 



The walls of the filaments, at least those comprising the segments, 

 are microfibrillar (Preston, 1974; Borowitzka and Larkum, 1977), the 

 microfibrils containing chains of [3-l,3-xylan (Miwa et al., 1961). The 

 chains within the crystalline xylan fibril are coiled in double helices 

 (Preston, 1968). A p-l,3-linked glucan is also present, in the ratio of one 

 glucan molecule to four of xylan, and may form the cortex of the 

 microfibril surrounding a xylan crystallite (Preston, 1974). These 

 investigations elucidate, as well as extend, early critical wall studies 



