152 Elementary Biology. 



so that the bast is outside and the wood inside, but so that 

 the two portions lie alternating with each other. Such an 

 arrangement is known as a radial strand. 



The roots, as a whole, originate from the base of the 

 flattened stem in the form of long branched or unbranched 

 filaments, each terminated by a root-cap. The cap may be 

 very well studied in the aquatic roots of the duckweed, an 

 aberrant monocotyledon which exhibits a root-cap not dif- 

 fering essentially in this respect from that of the monocoty- 

 ledon. The roots give origin to rootlets and root-hairs by 

 which the absorption of food from the soil is carried on. 



Leaf. The leaves of the lily are peculiar in being 

 sessile ; their bases are generally the widest parts, and, in 

 the case of the outer leaves of the bulb, enclose the entire 



FIG. 70. ROOT-CAP OF Lemna minor. 



6. 



d 



a, root-cap ; c, epidermis of the root ; b, growing cells. 



stem. The fibro- vascular strands, as already stated, are 

 arranged in a parallel manner, the veins running without 

 branching from end to end of the leaf or merging into a 

 vein which runs round the margin of the leaf. The micro- 

 scopic structure of the leaf must now occupy our attention 

 for a moment. Superficially the leaf is covered by an 

 epidermis composed of flat tabular cells. The upper epi- 

 dermis has numerous stomata; these are, however, much 

 more numerous on the under epidermis (especially of dico- 

 tyledonous leaves). The guard-cells are, as a rule, smaller 

 than the general epidermal cells, and are concavo-convex 

 in outline, the space between being the stoma. The guard- 

 cells contain chlorophyll, which is absent from the other 

 epidermal cells. The stoma leads into a large intercellular 



