1072 Stephens . — The Structure and Development of the 
taking place in the cells surrounding it. These divide up to form a mass 
of small transparent nucleated cells — the ‘ nucleus \ It is interesting to 
note that this appears to be formed entirely by the renewed division of 
cells derived from the original subhypodermal layer of the mother root. 
The two outer layers of the haustorium (derived from the two outermost 
layers of the root) merely divide to form a cortex several layers in thickness 
(<:., Figs. 1, 5, 6 ; in Fig. 5 the cortex is still only two layers thick in the 
lower portion of the haustorium). In the younger stages, as Fig. 5, this 
cortex is still clearly marked off from the small meristematic cells of the 
‘ nucleus \ but later on the outer cells of the latter enlarge, lose their slightly 
rounded outlines, and pass over into ordinary cortical cells (cf. Fig. 6). 
Around the axial strand, however, they seem to remain active until the 
death of the haustorium, and perhaps play some part in the transmission of 
elaborated food-stuffs from host to parasite. 
By the time the ‘ nucleus ’ is well formed, a line or group of papillae 
(those which were the first to penetrate the host root) have bored their way 
through or between the endodermal cells, and are now tapping the vessels 
of the stele. They penetrate to various depths ; in Figs. 6 and 7 one is 
just entering the large central vessel. Their ends flatten against the vessels 
as they did against the endodermal cells, and the ferment they secrete 
dissolves the wall of the vessel. Sometimes the papilla then grows on and 
penetrates through the opposite wall into the next vessel (cf. Fig. 7), some- 
times instead its tip becomes dissolved together with the wall, so that it 
forms a side pipe tapping the vessel. One vessel may be tapped in this 
way by a longitudinal row of papillae. Spiral thickenings run up the 
papillae to their tips, and the tracheides thus formed constitute an efficient 
conducting system between the xylem of the host and that of the parasite. 
The further development of the haustorium is the result of the continued 
division of the cells of the ‘ nucleus \ which causes the haustorium to grow 
down on either side of the host root till it may come to resemble an india- 
rubber ball with a stick (the maize root) pressing it in along one side. 
Several cases have been seen in which the lips of the haustorium had closed 
round the maize root and destroyed its cortex till its stele appeared to be 
almost embedded in the haustorium. 
Examination of a large range of material has shown that the course 
of development outlined above is followed in practically every case. The 
very few exceptions that have been seen will be discussed later in con- 
nexion with the comparison of the Striga haustorium with that of other 
plants. The development of haustoria on the underground shoot may be 
mentioned here ; this is occasionally seen when a maize root comes in contact 
with the shoot. No early stages of development of these haustoria have 
been found, but sections through several mature haustoria have shown 
them to be of the usual type, except that they are almost embedded in the 
