THE STEM. 



23 



hook. Then the whole tendril shortens by coiling up 

 spirally, thus bringing the plant nearer the support. The 

 Virginia Creeper ^^ 

 develops the ten- 

 dril-tips into adher- 

 ing disks when it 

 climbs walls or 

 smooth trees (Fig. 

 15). Other ten- 

 drils, as of the Pea, 

 etc., are modified 

 leaves instead of 

 branches. Spines, 

 or thorns, are sometimes stunted and hardened branches, 



as in neglected Pear-trees, Plum-trees, 



etc. (Fig. 16). 



22. Subterranean stems and branches 



may seem on cursory examination to be 



little, if at all, different from roots. 



But they are jointed, and roots are not ; 



they have modified leaves in the form 

 16 of scales — always absent in roots ; buds 



may generally be found on them, but roots never produce 

 buds. Four com- 

 mon types of sub- 

 terranean stems 

 are the Rhizome, 

 Tuber, Corm, and 

 Bulb. The rhi- 

 zome (Gr. rhiza. 



Fig. 15. Tendril, which ends in coils {t) and disks (rf) of the Ampelopsis, or 

 Virginia Creeper. Fig. 16. Spine or stunted branch of the Crab-apple. Fig. 17. 

 Rhizome (rA) of Mint : r, roots ; si, stem. 



