vn.] ANATOMY OF PARASITES 65 



forms two kinds of spirals. If it be growing 

 rapidly, the coils are loose and it fails to form 

 haustoria ; but when this period of rapid growth 

 is over, it coils in more horizontal spirals closely 

 adpressed to the host-plant. It is on these 

 alone that the haustoria are formed. The sucker 

 originates at a point within the cortex, where a 

 focus of " merismatic," or rapidly - dividing 

 embryonic tissue is formed. This invades the 

 central region of the stem of the dodder and 

 also extends outwards. Vessels are now formed 

 in this, and become united to the central cylinders 

 of both host and parasite. In dodder the 

 epidermal cells of the aerial stem become 

 " claspers," and then the sucker penetrates 

 through the middle of them. This fixation of 

 the cup -like clasper appears indispensable to 

 complete the development of the penetrating 

 sucker. For when the epidermal cells do not 

 prolong themselves into clasping organs, the 

 sucker which arises within them undergoes an 

 arrest of development, and does not form 

 vessels. From this it appears that either the 

 hairs are the vehicle of irritation, which must 

 be kept up, and by conduction influences the 

 deeper tissues, which in response begin to form 

 the absorbing organ from within ; or it may be 

 that the "purchase" required for the necessary 

 force to drive the sucker-root into the host is 

 insufficient. The epidermal cells between any 



