ELEMENTS OF BOTANY. xl 



23. A Runner is a thread-like prostrate branch, producing roots and a tuft of 

 leaves at its extremity. 



24. Spines, or Thorns, are imperfectly developed, leafless branches, with hard 

 tips. 



25. Tendrils are the thread-like spirally coiled branches of weak and slender 

 plants, by means of which they attach themselves to other and stronger objects 

 for support. Leaf-stalks and parts of the inflorescence are occasionally convert- 

 ed into tendrils. 



26. Plants which die down to the ground at the close of the season, or after 

 maturing seed, are called Herbs, or Herbaceous Plants. Those with woody stems, 

 lasting from year to year, when of humble size, are called Shrubs, and when 

 reaching an elevation of twenty feet or more, Trees. 



4. Internal Structure of Stems. 



27. The stems of Phaenogamous Plants are composed of cellular tissue, woody 

 tissue, and vessels ; and upon the arrangement of the latter are founded the two 

 divisions of Exogenous and Endogenous Plants. 



28. Exogenous stems consist of a central column, called the Pith ; an external 

 covering, called the Bark ; and a middle portion, called the Wood. 



29. Their Pith is a mass of cellular tissue, enclosed in a thin sheath of spiral 

 vessels, termed the Medullary Sheath. 



30. Their Wood is composed of one or more layers of woody and vascular 

 tissue, traversed by thin plates of cellular tissue, called the medullary rays, and 

 annually increased, in all perennial stems, by the addition of a new layer to the 

 outside of that of the previous year. The new wood is called the Alburnum, or 

 Sap-wood, and the older and harder portion, the Duramen, or Heart-wood. 



31. The Bark, like the wood, is made up of layers. The inner bark, or Liber, 

 is composed chiefly of woody fibre. Between it and the wood, in the growing 

 season, is secreted a thin mucilage, called the Cambium, in which the new layers 

 of wood and bark are developed. Surrounding the inner bark is the Green bark, 

 consisting of cellular tissue filled with Chlorophyll, or the green matter of veg- 

 etables. Covering the whole is a thin membrane of cellular tissue, called the 

 Epidermis, or Cuticle. 



32. Endogenous stems exhibit no distinction of pith, wood, and bark ; but 

 are composed of threads or bundles of woody tissue, irregularly embedded in 

 cellular tissue. They increase in diameter by the formation of new bundles, 

 which are chiefly directed to the centre of the stem. 



5. The Leaves. 



33. Leaves are expanded appendages of the stem, developed from axillary 

 and terminal buds. They consist of loose cellular tissue, supported by a net- 

 work of woody and vascular tissue, called veins or ribs, and protected by the 

 epidermis. In them the fluids received from the root, and what they imbibe 

 from the air, through minute openings in the epidermis, called stomata, are con- 

 verted into the proper food of the plant. 



34. In the bud, they are folded, plaited, or coiled in various ways. This is 

 termed their Vernation. 



