256 



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 fil)ro-vascular bundles, from 

 which they alssorb nourishment. 

 The haustorial function is simi- 

 lar in other parasites, including 

 the parasitic Fungi, though here 

 the al)Sorbing 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 convcnientl,y 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 cylind(>r. grows for 

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

 can shorten in length, especially in some ])erennial herbs, 

 which thus are kept below ground despite the yearly growth 



Fig. ISO. — Eneli.sh Ivy, 

 showing the aerial root^ by 

 which it cUngs to walls. (From 

 Le Maout and Deoaisne.) 



