94 
Peirce . — A Contribution to ' the 
enlargements in the zones where haustoria are most abundant. 
A section of the petiole at a point about midway between 
two of the flower-clusters which it bears, that is, at the line 
a-b in Fig. i, shows (Fig. i) that, under the single-layered 
epidermis (e) and the two or three layers of collenchyma, 
come many layers of cortical parenchyma-cells (c). Around 
three sides of the petiole this cortical parenchyma abuts upon 
the proportionally narrow phloem-portion (b) of the vascular 
tissues. The phloem consists mainly of sieve-tubes and bast- 
parenchyma, only small and scattered groups of bast-fibres 
being found. Separated from the phloem by a single-layered 
cambium is the broad xylem (#), composed of rather small 
ducts and, in much larger proportion, of thick-walled wood- 
fibres. At one side, bordering upon the phloem, is the small 
bundle (/) which runs to one of the leaflets. The vascular 
ring of the petiole, enclosing the pith (/) in wdiich are a mass 
of sclerenchyma-cells (/) and small scattered groups of scle- 
renchyma-fibres in the region marked (s), is incomplete on the 
upper side of the petiole, and here the cortical merges directly 
into the pith-parenchyma. An aborted haustorium is shown 
at H. The haustorium aborted probably because of the 
absence in this region of vascular tissues with which it could 
unite. After its abortion, and after the atrophy and fall of 
the parasitic stem which gave rise to it, this atrophy being 
hastened, though, I think, scarcely caused by the abortion of 
the haustorium, it became covered over by the epidermis of 
the petiole. Such aborted and partly or entirely covered 
haustoria are the only remaining traces of the former presence 
of a continuous stem wound around the petiole, and these are 
few in number. 
If one now compares this section with the one shown in 
Plate VIII, Fig. 3, which is at the line c-d in Fig. 1, one sees 
at once that the larger size of the petiole at this point is due 
to the increase in the number of layers of parenchyma-cells in 
the cortex (c), to the formation in it of two sets (/, s') of 
sclerenchyma-masses, to the greater width of the band of wood 
(x), in which the proportion of fibres to ducts is even greater 
