42 



STEMS. 



[section 6. 



prickles, such as those of Blackberry aud Roses, are only excrescences 

 of the bark, aud not brauclies. 



102. Equally strange forms of 

 stems are characteristic of the 

 Cactus family (Fig. 111). These 

 may be better understood by com- 

 parison with 



103. Subterranean Stems 

 and Branches. Tiiese are very 

 numerous and various ; but they 

 are commonly overlooked, or else 

 are confounded with roots. From 

 their situation they are out of or- 

 dinary sight ; but they will well 

 repay examination. For the veg- 

 etation that is carried on under 

 ground is hardly less varied or 

 important than that above ground. 

 All their forms may be referred to 

 four principal kinds : namely, the 

 Rhizoma (^Rhizome) or Rootstock, 

 tlie Tuber, the Conn or solid bulb, 

 and the true Bulb. 



104. The Rootstock, or Rhi- 

 zoma, in its simplest form, is 

 merely a creeping stem or branch 

 growing beneath the surface of the soil, or partly covered by it. Of 

 this kind are the so-called creeping, running, or scaly roots, such as those 



by which the Mint (Fig. 97), the Couch-grass, or Quick-grass, and many 

 other plants, spread so rapidly and widely, — " by the root," as it is s;iid. 

 Tiial these arc really stems, aud not roots, is evident from the way in which 



Fig. 95. A branching thorn of Honey Locust, being an indurated leafless branch 

 developed from an accessory bud far above the axil : at the cut portion below, tliree 

 other buds (a) are concealed luider the i)etiole. 



Fig. 96. Spine of Cockspur Thorn, developed from an axillary bud, as the leaf- 

 scar below witnesses: an accessory leaf-bud is seen at its base. 



Fig. 97. Rootstocks, or creeping subterranean branches, of the Peppermint. 



