MISTLETOE 223 



and penetrates the cortex of the host. The penetration 

 seems to be due to mechanical force and to take place most 

 easily through weak spots such as lenticels and cracks. 

 When it reaches the wood growth of the peg stops. At 

 this stage it consists of large-celled, pitted parenchyma, 

 but later a strand of wood vessels develops and connects 

 with the wood of the host. 



Only at the beginning of the second year do the first 

 two leaves of the young plant appear, and for the next year 

 or two growth is slow ; later it becomes more rapid, and the 

 familiar bush is formed with its forking branches and paired 

 leathery leaves. Meantime there arise from the neck of 

 the first sucker exogenous branches, which grow within the 

 cortex of the host plant. Their tips are formed of spreading 

 brushes of slimy cells. In the main they run along the 

 branches ; sometimes they grow crosswise for a time and 

 then bend anew in the longitudinal direction. They never 

 encircle and strangle the branches of the host. From these 

 branches secondary suckers descend to the wood. At 

 intervals adventitious buds are formed which burst through 

 the cortex and give rise to fresh bunches of mistletoe, 

 and at the same points new branches of the absorbing system 

 arise. The utilisation of the host plant is thus extended, 

 and a single seed may give rise to many bunches of mistletoe ; 

 cutting off the visible bunches does not free the host from 

 the parasite (Fig. 25). 



As the host branch continues to form secondary wood the 

 sucker would, in the course of a few years, be enveloped and 

 destroyed, were it not that it possesses a meristematic zone 

 at the level of the host's cambium, through the activity of 

 which it lengthens by just the amount that the host increases 

 in thickness. Increase in thickness also occurs at this 

 point, so that a longitudinal section through a branch 

 bearing an old mistletoe shows the wood penetrated to 

 different depths by numerous formidable pegs. At the 

 point of attachment of the bush the host and its parasite 

 usually have a club-like appearance. 



We have not referred the absorbing system of the mistle- 



