26 



LECTURE in. 



Its root-apex becomes turned away from the light to the branch, the radicle bores 

 through the cortex, and makes its way through the cambium to the wood; if its 

 apex is situated deep in the wood, later on, this is the result of the new woody 

 layers of the branch growing round the base of the root, while this becomes elon- 

 gated a little to a corresponding extent. Later on, as it seems, roots arising from 

 the base of the stem grow within the soft living cortex, and from these again lateral 

 roots arise on the side turned towards the wood of the host : they penetrate through 

 the cambium layer to the wood, are then surrounded by the younger, growing, woody 

 layers, becoming meanwhile elongated to a corresponding extent at the base, and 

 now constitute the so-called suckers of the mistletoe. Of these three forms of root, 

 only those which grow in a sinuous manner within the cortex, apparently, have a root- 

 cap, which, however, even here is not well developed. The anatomical structure of the 

 Viscum root allows little to be recognised of the structure so distinctive of true roots; 



FIG. 15. — Lower portion of the stem [a) of the Mistletoe {Viscum album), h the wood of the slioot- , 



axis ; i primary root ; ff roots growing in the cortex of the host-branch (c) ; £■ two buds arising from 

 these ; ce so-called Haustarta, roots which penetrate through the cambium into the young wood, and 

 become surrounded by it later; bb wood of host-branch (half cut across at dd) showing the annual 

 rings (nat. size). 



even the characteristic axial vascular bundle is considerably reduced, and the 

 adaptive peculiarity of the sucker is especially striking — its growing point is 

 converted into permanent tissue, while the growth in length, inconsiderable it is 

 true, is carried on at the part which lies in the cambium layer of the branch of 

 the tree. In spite of all this, no unbiassed person will hesitate to regard these green 

 organs of the mistletoe, even though devoid of a cap and anatomically abnormal, as 

 reduced roots, and this is so far of some importance, since here we have one 

 of the first stages of reduction produced on roots by parasitism. Finally, it may 

 be mentioned that the Viscum roots losing themselves in the cortex of the host, repre- 

 sent very active organs of multiplication : from them spring shoots, which, breaking 

 through the cortex of the tree, come forth into the light, and from these, new roots" 

 then again run into the cortex, so that as occasionally occurs on old apple-trees 

 the whole tree, from crown to root, is infested with the mistletoe. 



