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A TEXTBOOK OF BOTANY 



[Ch. V, 8 



upon the trunks of Tree Ferns, which they sometimes cover 

 completely with their stiff brown threads, though the signifi- 

 cance of their presence is not clear. 



A very striking modification of roots is found in the absorb- 

 ing roots, or haustoria, of parasites. The common Dodder, 

 for example, a parasitic twiner (Fig. 59), puts out these 

 roots wherever it touches the host plant, and they, by use of 



digestive enzymes, penetrate the 

 host, and attach themselves to 

 the fibro-vascular bundles, from 

 which they absorb nourishment. 

 The haustorial function is simi- 

 lar in other parasites, including 

 the parasitic Fungi, though here 

 the absorbing structures are my- 

 celial threads, not true roots. 



Roots are transformed to spines 

 in some Palms, though the signifi- 

 cance of these structures is here 

 no plainer than in case of leaves 

 and stems (pages 79, 192). Also, 

 some other, though minor, special 

 functional uses and structural 

 modifications of roots have been 

 described. 



Though not matters of function, certain other special 

 matters about roots may conveniently be noted at this 

 place. Thus roots, like some stems, show anomalies of 

 structure often very puzzling, as, for example, the appear- 

 ance in Beets which simulates annual rings. These rings are 

 due to the fact that the cambium, after forming a certain 

 amount of phloem and xylem, ceases to grow ; thereupon a 

 new cambium arises just outside the first cylinder, grows for 

 a time, is itself replaced, and so on, many times. Again, roots 

 can shorten in length, especially in some perennial herbs, 

 which thus are kept below ground despite the yearly growth 



Fig. 180. — English Ivy, 

 showing the aerial roots by 

 which it clings to walls. (From 

 Le Maout and Decaisne.) 



