1070 Stephens . — The Structure and Development of the 
appearance characteristic of the developing root, which has here an ordinary 
endogenous development. Numbers of young lateral roots and haustoria 
have been seen arising side by side on the mother root, and it is clear that 
there is no resemblance between them in origin, development, or structure. 
(c) Connexion with Host. 
Penetration of the soft cortical tissues of the maize root is rapidly 
effected when once the haustorium comes into contact with it. Stages 
in which the tip of the haustorium has not already penetrated to the endo- 
dermis are rarely found. When the epidermal cells of the haustorium touch 
the host root they begin to elongate, forming papillae of a finger-like form 
and arranged in palisade rows, which bore their way between and through 
the cortical cells of the host. Their passage is effected by a ferment they 
secrete, which swells and ultimately dissolves the cell-walls which it touches. 
The margin of the advancing haustorium is marked by an irregular clear 
bright yellow or yellow-brown line, presumably this ferment plus the 
dissolved tissues of the host, and the same yellow line is often seen round 
individual papillae which have grown beyond the level of the rest of the 
haustorium. This line is represented (marked f.) in Figs. $- 8 . It 
stains with safranin, but not with haematoxylin ; microchemical tests to 
determine its nature have yielded no results. The walls of the cells of the 
host to a distance of several cells from the haustorium may become swollen 
with the action of the ferment, when they also take on this yellow colour 
and stain in a similar way (cf. the walls of the endodermal and cortical 
cells bordering the line of ferment in Fig. 7). The pressure due to growth 
assists in the passage of the haustorium through the tissues of the host, but 
to a much less degree, as is shown by the fact that very little collapsed 
tissue is seen in the path of its advance. Growth pressure is in evidence 
when individual papillae have to penetrate a hard-walled vessel, as in Fig. 7, 
where a papilla has evidently had considerable difficulty in penetrating into 
the central vessel of the maize stele. The wall of the latter has been pushed 
in, and the length of time that the papilla has taken to effect an entrance 
can be seen by the spiral thickening formed nearly down to its tip. 
A series of longitudinal sections through a haustorium soon after 
penetration (or an optical section such as Fig. 5) shows that the central 
papillae have reached the endodermis, and have flattened their ends against 
it to form several regular palisade layers. With regard to Fig. 5, it must 
be remembered that it is a median optical section ; focusing slightly above 
or below the level of the section, the papillae in front of and behind the 
layer figured as abutting on the endodermis can be seen as it were crawling 
over its surface trying to effect an entrance. As soon as the ferment touches 
the endodermis, the contents of the cells of the latter disappear, and their 
outer walls swell (Fig. 7). In Fig. 8 is seen an endodermal cell in longitu- 
