INTRODUCTION XIX 



Stems may be woody or herbaceous, the former being chiefly 

 characteristic of perennial plants. A woody plant with one main 

 stem at least ten or twelve feet high is called a tree ; whilst if it 

 branches freely near the ground it is a shrub, or if less than three 

 feet high, an undershrub. 



A large number of plants, known as herbaceous perennials, have 

 perennial undergound stems, but send up branches above ground 

 that are annual, dying down each winter. 



Aerial stems may be erect, prostrate, or ascending, horizontal, 

 that is, at first, but bending upwards at their points. They may 

 be twining, as in the Hop, Honeysuckle, and Convolvulus ; or may 

 climb in other ways, such as the roots in the Ivy, the prickles in 

 Roses, the tendrils in Tares, and the twisted leaf-stalks in Clem- 

 atis. They may be spinescent, ending in straight spines, as in the 

 Blackthorn, or they may exceptionally be flattened and leaf-life, as 

 in Butcher's-broom (p. 485). 



The runner is a prostrate axillary branch, rooting at its nodes 

 and bearing buds which develop into new plants, as in the Straw- 

 berry. The offset is similar but shorter, and bears only a terminal 

 bud, as in the House-leek (p. 176). 



The Leaf is most important as a means of distinguishing closely 

 allied plants. Underground stems and the aerial stems of a few 

 plants, such as the parasitic Toothwort, have small scaly or mem- 

 branous leaves ; but a typical leaf has a blade, a stalk, and a sheath, 

 or two appendages at its base known as stipules. Leaves which 

 have no stalks are termed sessile (sitting), as in Eryngo (p. 199). 



The stipules may be united round the stem, as in the Knot- 

 grass family, in which, and in the Rose, 

 they occur exceptionally in addition to a 

 sheath. 



Other characters of the leaves are their 

 vernation (from the Latin ver, spring), or 

 folding in the bud, their position and 

 arrangement, veining, form, base, apex, 

 margin, surface, texture, colour, and 

 duration. In vernation leaves may be 

 conduplicate, or folded down the midrib, Lea f of Rose with stipules : 

 like the two halves of a sheet of note- *> *p* x of the sheath. 

 paper, as in the cherry ; plaited, like a fan, as in the Beech ; 

 convolute, or rolled up like a scroll, as in the Plum ; involute, with 

 the margins rolled upwards, as in the Water-lily ; revolute, with 

 them rolled backward, as in the Dock ; valvate, when they touch 

 one another without overlapping ; or imbricate, where they overlap 

 like roofing-tiles. 



