BOTANICAL SUBJECTS. 167 



The tap root of a potato aj)pears to be no other than an under- 

 ground stem, which develops branch buds from its irregular nodes, 

 with or without tubercles, at the same time that it develops roots 

 from those nodes. Whenever one of these underground branches 

 lengthens and reaches the surface, it develops into an above-ground 

 stem, \\ii\i leaves instead of roots from its nodes. Other under- 

 ground branches may be arrested, at various stages, and do not 

 reach the surface of the ground, but remain, as storage stems 

 (potatoes) for use next season. The young underground shoots 

 of the potato are hairy, like young roots proper. 



In the mistletoe, the root end of the caulicle penetrates the 

 bark of its foster parent, and incorporates itself with the sap-wood. 

 The incorporation is like that which takes place in grafting and 

 budding. 



In some of the lowest plants the root is simply what is called a disk 

 of attachment, as in the root of Cystophora Sonderi (Fig. 43). In 

 this the fronds are leaves and roots in one, while the root proper 

 is only a disk of attachment. 



Fig. 43. Disk of Cystophora Sonderi, T. Ag. (pi. 243, "Han^ Phyc. Austr.")- 



As long as plants remained small, the disk was quite sufficient 

 for their anchorage. When, however, the plant became more com- 

 plicated, compound, and weighty, owing to its colonial extensions, 

 a larger anchorage, -vvith a broader base was needed, in order to 



Fig. 44. Root of Cystophora paniciilata, T. Ag., pi. 247, O.C. 



