i8o LIFl:: : OUTLINHS OF GENERAL BIOLOGY 



is a parasite on South American cactust*s of the genus Cereus, plants 

 remarkably well adapted to making the most of the scanty water- 

 supply in deserts. Now in South Africa there is another leafless 

 nustletoe. I'lstiiwi minimum, which grows on a spurge, Euphorbia 

 pohiiona. that Ixars a very close resemblance to the unrelated 

 cactus! 



In his luminous essiiy on Parasitic Plants ( !()_'()) Macgregor Skene 

 makts an ujstructivt- comparison between the growth of the mistletoe 

 on an appl« -tree and the growth of a scion on its stock, say, of a 

 pippin on a crab-apple. The differences are ]xrhaps greater than the 

 rest-mblances. A mistUtoe cannot be grafted on a host; it must 

 tstablish tlu- linkage in its own way. Grafting is successful only 

 whrn scion and stock are nearly related, e.g. apple and pear; but 

 the n)istlel(H' grows on an unrelated tree. The scion must get the 

 recpiisite wattr and .salts from its stock, but it gives back organic 

 f(M)d; the mistletoe gives back nothing. "The exixTiment has been 

 made of growing a mistletoe on a young apple-tree and, when it was 

 fully establislied. of removing all the apple foliage. For a year the 

 union subsisted, and then the apj^le diicl from starvation and with 

 it the misthtoe." 



TilK DoDDiK.- In contrast to the mistletoes, and doubtless 

 arising in a very diffcnnt way. are the dodders (Cuscuta),in which 

 there is no chlorophyll. They are related to the Bindweeds, and are 

 to Ix' thought of as climlxTS which have become parasites. A reddish 

 British s|xci«s is found growing in a tangle with ling and thyme; 

 anotlHT of a yrllowish colour victimises clover and flax. It often 

 deserves its German name of "devil's twine". Tlie foliage is repre- 

 sentee! by a frw minute scales, corresponding to leaves reduced 

 almost to vanishing ix)int. Except in the seedling there is no root. 

 But the long stem has lx)th activity and sptcialisation, and at 

 frerjiunt intervals it Ix-ars clusters of small Ixll-like flowers. 



rh«- s^M'ds of the clover's dodder fall to the ground in autumn, 

 or thev may Ix' sown along with thos*- of the harvested crop. In 

 either cas<> they do not grrminati- till there are young clover plants 

 rising from tin* ground. Out of the s<cd there emerges a slender 

 yellow thread, an inch or two in length, with a vestigial root at one 

 end and a mobile shcmt at the other. The root end may absorb 

 water, but it does not entir the gromid; the other end grows at its 

 own ex|x*ns<\ shrivrlling Uhind. anrl it nutates like a twining plant. 

 It levers its<lf on the groimd as if it were a little threadworm seeking 

 for a host It sometimes survives alxiut a month of starvation. If it 

 tr)uchrs a clover stmi. it twines roimd it. making several close coils, 

 from the inner surface of which there grow small suckers or haustoria, 

 jwrntrating into the bast-tissue of the host. In this way the young 

 df)ddrr gets materials for further growth, and it Ixgins to branch. 

 I.oos<; coils arc formed rounri the stems and leaf-stalks of the clover, 



