Germination of A r cent ho bium Occident ale. 109 
cortex into the medullary rays of the host, traversing the cambium and 
passing into the wood. In Fig. 23, a diagram of part of a cross-section 
of an infected pine branch, we see that from a vascular bundle of the 
parasite (shaded) in the cortex of the host (unshaded) a vascular bundle 
occupies the middle of the branch of the parasite which has penetrated into 
the wood of the host through a medullary ray. Figure 24 represents a 
tangential section of an infected pine branch through the wood, and shows 
the direct contact of a tracheid of the parasite (shaded) with a tracheid of 
the host. Figure 25 is also a tangential section of a branch of pine, but 
through younger wood, and shows, in the centre, the strand of tracheids 
and vessels connecting with the older wood, and at the sides the direct con- 
nexion of younger Arceuthobmm cells with the young tracheids of the host. 
At a in this figure we see how thin is the membrane through which the 
young Arceuthobium cell absorbs water from the pine tracheid. By 
preserving these thin places in the walls, the transfer of water from the 
cells of the host to the adjacent cells of the parasite is made as easy 
probably as the transfer of water from tracheid to tracheid through the wood 
of the host. I have seen no cases in which no membrane at all intervened 
between the tracheids of host and parasite, though there may be such 
places. 
In this perfect connexion of the xylem-tissues of parasite and host we 
have nothing uncommon. A more interesting question is as to the relations 
of the phloem-tissues of the two. In tangential and other sections through 
the bark I have sought vainly for anything more than contact between the 
cells of Arceuthobium and the sieve-tubes of pine* Fig. 2 6 at a and b 
shows Arceuthobium cells (shaded) in direct contact with sieve-tubes of 
pine. This tangential section was so thin that under an oil-immersion 
objective I could plainly see the sieve-plates in actual section, but I could 
never find any place where the pine formed a sieve-plate on the side of the 
tube toward an adjacent Arceuthobium cell. There may be such places, 
but I have searched long and carefully enough to doubt it. In this respect, 
therefore, Arceuthobium resembles other chlorophyll-containing parasites, 
e. g. Viscum , Fhoradendron (Peirce, 1893). But from the anatomical fact 
of the absence of sieve-plates in the walls between the sieve-tubes of the 
host and the living cells of the parasite the inference is not justified that 
the parasite absorbs only food-materials, not elaborated foods, from the 
host (Pfeffer i. 355 ; Peirce, 1903). In fact Fig. 2 6 shows plainly that 
osmotic transfer must take place through the thin walls separating the 
sieve-tubes of the host from the cells of the parasite whenever the physical 
conditions make osmotic movement through the thin walls in either 
direction necessary. The presence in either host sieve-tube or parasitic cell 
of a greater proportion of dissolved substance, food or other, will inevitably 
entail the movement of that substance to the cell that contains less. 
