22 ELEMENTS OF STRUCTURAL BOTANY. 



Shepherd's Purse. This plant, (Fig. 2G). is one 

 of the commonest of weeds. As in the Buttercup, the 

 foUage-lcaves are of two kinds, radical and cauline, the 

 former heing in a cluster around the hase of the stem. 

 The cauline leaves are all sessile, and each of them, at 

 its base, projects backward on- each side of the stem, so 

 that the leaf somewhat resembles the head of an arrow. 

 Such leaves are, in fact, said to be sayittate, or arrow- 

 shaped. The flowers grow in a cluster at the top of the 

 stem, and, as the season advances, the peduncle gradu- 

 ally elongates, until, at the close of the summer, it 

 forms perhaps half of the entire length of the stem. 

 You will observe, in this j^lant, that each separate 

 flower is raised on a little stalk of its own. Each of 

 these little stalks is a pedicel, and when pedicels are 

 present, the term peduncle is applied to the portion of 

 stem which supports the whole cluster. 



28. The flowers, (Fig. 27), are rather small, 

 and so will require more than ordinary care in 

 their examination. The calyx is polysepalouf, 

 and of four sepals. The corolla is polypetalous. 

 Fig. 27. and of four petals. The stamens, (Fig. 28), are 

 six in number, and if you examine them atten- 

 II tively, you will see that two of them are shorter 

 than the other four. The stamens are conse- 

 quently said to be tetradjfnamous. But if there 

 Fig. 28. liatl been on\y four stamens, in two sets of two 

 each, they would h-ave been called didynamous. The 

 stamens are inserted on the receptacle (hypogynous). 

 The pistil is separate from the other parts of the flower 

 (superior). 



29. To examine the ovary, it will be better to select 

 a ripening pistil from the lower part of the peduncle . 



