THE STEM. 27 



in their skins, reserves the most nutritive parts for the pigs. 

 All the swollen underground stems just spoken of are especially 

 characteristic of dry climates, for which they are well suited, as 

 the condensed form offers comparatively little surface from which 

 evaporation can take place. 



Modified Stems. — Hitherto we have had to do with typical 

 undeniable stems, easily recognizable as such to the ordinary 

 observer, except perhaps stem -tendrils. There are, however, 

 numerous cases where stems are so modified, for the purpose of 

 performing special functions, that they can only be recognized by 

 homology. In other words, relative position and mode of develop- 

 ment must be taken as criteria, and not the functions performed. 

 This has already been spoken of on p. 7. Let us apply this to 

 stem-tendrils. A direct continuation of an organ is evidently of 

 the same nature as that organ. In the vine the youngest tendril 

 is a continuation of the stem, and may therefore be regarded as 

 part of it. This was previously the case with the youngest 

 tendril but one, which, however, has been pushed on one side, its 

 place being taken by a new stem borne in the axil of the youngest 

 leaf. And so on for the next tendril. Thus, in any branch, the 

 tendrils taken in succession, commencing with the oldest, have in 

 turn occupied the end of that branch. We have therefore to deal 

 with a sympodium (see p. 24). Another reason for not regarding 

 the axis of a vine branch as a simple stem bearing branches in the 

 usual way, is the fact that the tendrils do not grow from the axils 

 of leaves. Examination of passion-flower tendrils shows that 

 these do grow from leaf-axils, and we are therefore justified in 

 considering them to be branches. Vine tendrils are further 

 regarded as modified flower-stalks, because all possible gradations 

 are found between the two. Stems may also bear branches modi- 

 fied into spines and thorns for protective purposes. These are 

 known to be of stem nature from their axillary development and 

 the presence of leaves upon many of them. It sometimes happens 

 that stems undergo modification owing to the fact that the leaves 

 are very small or absent. This means that some of the functions 

 of the ordinary leaves have to be carried on by the stem, which 

 in this case may be called a plujlloclade. In cacti, for example, 

 apart from the flowers, the leaves are reduced to minute spines, 

 and the green stem assumes the most remarkable forms, globular, 

 jointed, prismatic, &c, all of them very compact, and suited for 

 dry climates (see above). In other cases, as asparagus and 

 butcher's broom (Ruscus), branches assume a flattened, leaf -like 

 form, and are liable to be mistaken for leaves. Such phylloclades 

 are termed cladophylls or cladodes. The stem of duckweed may 

 perhaps best be placed in this category. 



