i88i.] BOTANY FOR GARDENERS. 29 



gardens entails a large amount of labour — much more so than if trench- 

 ing, manuring, &c., only were needed. Trenching and thoroughly mix- 

 ing the soil takes almost double the time of ordinary trenching. Never- 

 theless it pays well in the end, when satisfactory crops can be produced 

 with some certainty. When the soil is once got into fair condition, it 

 is not difficult afterwards to maintain it in order, and continue to im- 

 prove it. Under judicious management it should never require to be 

 all trenched the same season, but a portion at a time, thus extending 

 the operation over a series of seasons. Wm. Bardney. 



BOTANY FOR GARDENERS. 



NO. III. — STEMS. 



The Stem or Trunk is that portion of the tree which has by some been 

 termed the axis, supplied with pipes, cells, and filters, and through 

 which the sap rises in its progress to the leaves. Part of the stem 

 displays a vascular, and the other portion exhibits a cellular, structure. 

 The stem is produced by the successive development of leaf-buds, 

 which cause a corresponding horizontal growth between them, and 

 varies in structure in four principal ways, — viz., Exogens, with 2 

 cotyledons, the radicle itself usually elongating ; leaves net-veined ; 

 perianth (petals) in 4's or 5's ; wood in a continuous ring, formed by 

 successive additions to the outside. Endogens, with 1 cotyledon, the 

 radicle usually remaining undeveloped ; leaves straight - veined ; 

 perianth in 3's or multiples of 3's ; wood in isolated bundles, formed 

 by successive additions to its centre. Acrogens, when the stems are 

 formed by the union of the bases of the leaves, and the extension of 

 the point of the axis ; or by simple elongation or dilatation, where no 

 leaves or buds exist, as among Thallogeris. In altitude or length, and 

 diameter, stems present the most varied and contrasted features. 

 According to some travellers, there is a Palm that grows 15 feet high, 

 with a trunk not thicker than a finger. A comparison, indeed, be- 

 tween the stems of various plants would in some cases afford examples 

 of widely divergent extremes. The Scirpus capillaris is not thicker 

 than a hair, and some are as fine as a gossamer thread ; while the trunk 

 of the Baobab is nearly 100 feet in circumference. The stem of 

 Exogens may be distinguished into the Pith, the Medullary Sheath, 

 the Wood, the Bark, and the Medullary Rays. 



The Pith consists of cellular tissue, occupying the centre of the 

 stem. It is always solid when first organised j but in some cases it 

 separates into regular cavities, as in the Walnut, when it is disciform ; 

 or it tears into irregular spaces, as in the Umbelliferas (Parsley, Carrot, 

 and Parsnip, for instance). 



The Medullary Sheath consists of spiral vessels ; it immediately 

 surrounds the pith, projections of which pass through it into the 



