DICOTYLEDONS. 173 



ful examination of a typical dicotyledon, and a comparison 

 with this of examples of the principal orders and families. 



One of the commonest of weeds, and at the same time one 

 of the most convenient plants for studying the characteristics 

 of the dicotyledons, is the common shepherd' s-purse (Oapsella 

 bursa-pastoris) (Figs. 93-95). 



The plant grows abundantly in waste places, and is in flower 

 nearly the year round, sometimes being found in flower in 

 midwinter, after a week or two of warm weather. It is, how- 

 ever, in best condition for study in the spring and early 

 summer. The plant may at once be recognized by the heart- 

 shaped pods and small, white, four-petaled flowers. The plant 

 begins to flower when very small, but continues to grow until 

 it forms a much-branching plant, half a metre or more in 

 height. On pulling up the plant, a large tap-root (Fig. 93, A, r) 

 is seen, continuous with the main stem above ground. The 

 first root of the seedling plant continues here as the main root 

 of the plant, as was the case with the gymnosperms, but not 

 with the monocotyledons. From this tap-root other small ones 

 branch off, and these divide repeatedly, forming a complex 

 root system. The main root is very tough and hard, owing to 

 the formation of woody tissue in it. A cross-section slightly 

 magnified (Fig. 93, M ), shows a round, opaque, white, central 

 area (#), the wood, surrounded by a more transparent, irregu- 

 lar ring (ph.), the phloem or bast; and outside of this is the 

 ground tissue and epidermis. 



The lower leaves are crowded into a rosette, and are larger 

 than those higher up, from which they differ also in having 

 a stalk (petiole), while the upper leaves are sessile. The 

 outline of the leaves varies much in different plants and in 

 different parts of the same plant, being sometimes almost 

 entire, sometimes divided into lobes almost to the midrib, and 

 between these extremes all gradations are found. The larger 

 leaves are traversed by a strong midrib projecting strongly on 

 the lower side of the leaf, and from this the smaller veins 



