30 PRACTICAL GUIDE TO GARDEN PLANTS 



called the ' cambium layer.' As long as ever a Dicotyledonous plant lives, 

 this cambium layer has the power of doing two wonderful things : it 

 adds by division of the cells a layer of wood on the inside every year, 

 and a layer of bark on the outside, and it is by this process, which 

 goes on year after year in woody stems, that the latter increase in bulk. 

 It thus happens that the youngest part of the wood of a tree trunk is 

 on the outside beneath the bark, and not in the centre near the pith ; 

 while the youngest bark is next to it and not on the extreme outer 

 surface. It is the addition of a layer of wood each year to that already 

 existing that gives the ringed appearance to tree trunks, each ring 

 representing one year's growth. 



If a strip of bark be taken off all round the trunk or branch of a 

 tree without injuring the wood, it will be found that the leaves do not 

 shrivel up or wither, as one might expect, as a result of the operation. 

 It is evident, therefore, that the sap from the roots ascends by the 

 vessels in the cambium and young wood, and not by means of the 

 bark, nor yet by means of the pith or the old inner wood, as is indeed 

 obvious when one sees a huge Elm, Willow, or Oak with all the 

 interior scooped out of the trunk. It will also be noticed that, if a 

 branch has a string tied round it firmly, the portion above the 

 string will become swollen. This shows that the elaborated or assimi- 

 lated food made by the leaves returns dovm the stem by the outer cells, 

 and as these are compressed at the tie the descent of the nutritive 

 material is checked at that point and the cells above become gorged. 



There are many other kinds of cells and vessels in the stems of 

 Dicotyledonous plants, some being spiral, like compressed watch-springs, 

 some cylindrical with slits or holes in the sides, or only at the base 

 where they join another vessel, and so on ; and running through them all 

 from the central pith or ' medulla ' to the circumference are rays known 

 as ' medullary ' rays, popularly known as the ' silver grain ' in wood. 



All the cells and vessels in a stem are not of the same nature : some 

 are very tender like those of the cambium, some tough like those 

 of the bast cells, and others fibrous or woody. The latter are seen in 

 the principal nerves or veins of leaves, and are gathered together into 

 bundles. Each leaf is connected with the stem by means of these 

 fibrous bundles, and as the leaves are arranged all round the stem it is 

 obvious that the fibre-bundles from them collect and form a circle 

 round the stem. The traces of these fibre-bundles from the leaf into 

 the stem are well seen in the scars left by the fallen leaves in autumn, 

 as in the Horse-Chestnut and other trees. 



Such are briefly the main points in the structure of the stems of 

 Dicotyledonous plants, and they are referred to here chiefly because a 



