STEMS. 35 



of the characters by which the two leading divisions 

 of the Vegetable Kingdom are characterized. Thus 

 in one of these divisions the veins are branched, so 

 as to form a network throughout the leaf; the 

 plants with these netted veins, to which belong our 

 trees, shrubs, and most herbs, are the Dicotyledons, 

 or Ex'ogens ; while in the second division, the veins 

 run parallel to each other, being little or not at all 

 branched, and not forming a network. The plants 

 with parallel veins, among which are our grasses, 

 lilies, &c., are the Monocotyledons or En'dogens. 



Stems. In the stems of plants, the tissues are 

 arranged round a centre; otherwise, in the sim- 

 pler and lower plants, they agree in structure with 

 leaves, the centre being occupied by some element 

 of nbro-vascular tissue, as simple wood-cells, a few 

 vessels or ducts. 



In the higher or flowering plants, the stem exists 

 in two distinct forms, corresponding to the differences 

 above noticed in the arrangement of the veins of the 

 leaves ; these must be considered separately. 



In the Dicotyledons or Exogens (PI. I. fig. 36), the 

 centre of the stem, in a transverse section, is seen to 

 be occupied by the pith or medulla, which is repre- 

 sented in the figure by the innermost circle. Imme- 

 diately outside and around this is a narrow ring, in- 

 dicating the section of a sheath to the pith, and called 

 the medullary sheath. Next comes a broad ring of 

 wood of the first year's growth (fig. 36 a), traversed, 

 from the pith to the bark, by wedge-shaped paler 

 rays, termed the medullary rays. Outside the first 

 year's wood is the newer and paler wood of the second 

 year (b] ; and so on, a new ring of wood being added 

 outside the preceding layer for each year of growth 

 of the stem. 



On the outer side of the wood is the inner bark or 

 liber (fig. 36 c) ; and outside this is the spongy outer 

 bark (d), covered by its epidermis. 



