298 Peirce.— On the Structure of the Hazes toria 
impaired, and they are correspondingly less nutritious to the 
parasites. It is noticeable that, so far as the material at hand 
is concerned, the parasites living on these plants are smaller, 
and produce fewer flowers, than those growing on the other 
two species. There may be reasons other than the structure 
of the hosts, and the consequent mode of growth of the haus- 
toria, for the weakness of the parasites. The other reasons 
are unknown ; this reason is evident. 
While the sucker has been making its way through the 
parenchymatous tissues of the host to the fibro-vascular bun- 
dles, and while the rest of the haustorium has grown at the 
same rate with it, certain changes have been taking place in 
the shaft of the haustorium and in the parasite itself. If one 
makes a tangential section of a Cuscuta - stem so that it shall 
pass through a haustorium already imbedded at its distal end 
in the tissues of a host, at .right angles with the long axis of 
the haustorium, one finds that the shaft of the haustorium is 
no longer composed of parenchymatous tissue only. If the 
section be made near the surface of the Cuscuta - stem it is seen 
that the haustorium consists of two concentric layers of tissue 
which are unequal in thickness (see Fig. 10). The outer or 
cortical layer is composed of fairly compact, parenchyma- 
tous, angular cells. This cortex abuts, without the interposition 
of a sharply-defined epidermis, upon the loose cortical paren- 
chyma of the mother-plant, those cells immediately surrounding 
the haustorium being more or less compressed by its growth. 
Within this zone of cortex, and about one half as thick, is the 
central cylinder, composed at first of procambium-cells. These 
more or less rapidly and completely differentiate into three or 
four, sometimes more, fibro-vascular bundles, at first separate, 
but later confluent. The central procambium-cells become 
large, thick-walled, lignified, evidently ducts or tracheids (see 
Fig. 11, t). One or two layers of cells on either side of these 
remain merismatic for a time, and form, therefore, a temporary 
cambium (Fig. 11, c). Immediately outside these two cam- 
bium-layers are two masses of soft bast, consisting of cells of 
large and small diameter in varying proportions (Fig. 11,/). 
