282 PARASITISM IN PLANTS AND ANIMALS. 



a suitable place for its development. Circumstances like these 

 have doubtless initiated the complex phenomena of parasitic life, 

 and creatures so situated have adapted themselves to altered con- 

 ditions by corresponding, but gradual alterations of their own 

 structure. Some examples of the changes which have been so 

 brought about I shall have briefly to point out to you this 

 evening. 



Let us, in the first place, glance at some of the phenomena of 

 parasitic growth in plants. If we look at the stem of a forest 

 tree entwined by a beautiful growth of ivy, we naturally get the 

 idea that the ivy derives its life from the encircled tree. This 

 is not so. Those familiar root-like fibres which are given off so 

 abundantly from the ivy-stem are not roots in the proper sense 

 of the term ; they draw no juices from the supporting tree, but 

 are mere anchors or hold-fasts : a stone wall will serve them as 

 well as a tree-stem. The vivid green of the ivy is of itself suffi- 

 cient to contradict the idea of its parasitism, for in perfectly 

 parasitic plants the green colouring matter gives place to brown 

 or some neutral shade. The presence of chlorophyll or green 

 material in leaves is not only refreshing to the eye, but it is of 

 the utmost importance to the whole creation, animal as well as 

 vegetable. It is the part of this substance to decompose the car- 

 bonic acid of the atmosphere under the influence of sunlight, 

 setting free oxygen, and fixing the carbon in the tissues of the 

 plant itself. On this decomposition depends almost entirely the 

 growth and well-being of the plant, and seeing that animals are 

 dependent for the means of subsistence either directly or indi- 

 rectly upon the vegetable world, it is clear that without chloi'o- 

 phyll there would be a general collapse of life upon the earth. 

 But this function of chlorophyll the confirmed parasite is inca- 

 pable of performing. The degree of parasitism attaching to any 

 particular plant or animal may be pretty accurately estimated by 

 noting the amount of structural degradation which it has under- 

 gone. In plants the chief signs of these are the absence of green 

 colour, and the disappearance of leaves and of woody or vascular 

 tissue. In the mistletoe, for instance, which is more than half 

 parasitic in its habit, the leaf-green has undergone much altera- 



