■IAS 



TEXTBOOK OF PLANT PHYSIOLOGY 



trees. They send haustoria under the bark, attaching them to the 

 vascular system of the host and absorbing water and mineral salts. 

 Having well-developed and normally functioning leaves, they do 

 not need to obtain carbon compounds from the host. As a rule, 

 mistletoes do not harm trees very greatly, although they do take 

 away from the host part of the mineral and especially the nitro- 

 genous substances (Fig. 94). Hence, when several clusters of mis- 

 tletoe have settled upon a tree, it may eventually die. It is inter- 



Fig. 93. — Mistletoe on branches of a tree (after Transeau). 



esting to note that some trees are immune to mistletoe. The 

 seeds of the parasite which happen to fall upon their bark die soon 

 after germination. The cause of this immunity is not known. 

 Others, on the contrary, are unable to withstand the invasion of 

 this parasite. 



A very peculiar group of semiparasites are the insectivorous 

 plants. In the quest for the necessary nitrogen compounds they 

 have developed the ability of securing nitrogen from insects and 

 other small animals. They must, of course, first catch and then 

 digest these insects. The organs of capture have a rather varied 

 form and structure. In the sundew (Drosera), common in peat 

 bogs, there occurs a secretion of a sticky fluid on special tentacles, 



