36 Commercial Gardening 



cauiicle, which means little stem. Structurally it is mado up of cells, 

 fibres, and vessels, built up in the form of tissue characteristic of a stem. 

 Its functions are to hold up the seed leaves to the light and supply them 

 with water and food materials. Between the seed leaves the first bud of 

 the plant, known as the plumule, will be noticed. It is really the apex 

 of the young stem, covered with the rudiments of the first true or rough 

 leaves. Many variations are met with amongst seedlings. For instance, 

 in the Scarlet Runner, Broad Bean, and Oak the seed leaves remain in 

 the seed, below-ground, during and after germination. In these cases the 

 cauiicle remains very short, the seed leaves do not make their appearance, 

 and the plumule is the first part to rise above ground. The cauiicle under- 

 goes modification in other ways in certain plants. The upper portion of 

 the tuberous swelling of the Turnip and Radish consists of the cauiicle, 

 enlarged and fleshy, to serve as a store for reserve food. 



The Growth and Thickening- of the Stem. As the plumule grows 

 and develops into a stem of some length in the Stock or China Aster, it 

 is seen to be self-supporting, because the thickness and woody matter in 

 the interior is proportionate to the height. The leaves of the Cabbage 

 are much larger, and the stem becomes greatly thickened to support them. 

 The stem of the Gourd becomes enormously lengthened in proportion to 

 its thickness, but has to lie on the ground unless supported. If the stem 

 of any of these plants is cut across it will be found to have a core of pith, 

 consisting of thin-walled cells, surrounded by a layer of wood of greater 

 or less thickness, and that again by a bark of no great thickness, and 

 covered on the outside with a skin or epidermis. Such sterns of the first 

 year are usually green, because they contain chlorophyll, and are capable 

 of manufacturing plant food. The skin of green stems also has air pores, 

 or stomata, such as leaves have, and takes in oxygen from the atmosphere 

 for the purpose of breathing. The above, in general terms, is the structure 

 of a stem of one season's growth. 



In order fully to understand the thickening of a stem it will be neces- 

 sary to consider the structure of a shrub or tree, say the stem of an Apple 

 tree of some size. If the stem is cut across, the pith (fig. 20) will be found 

 in the centre, and probably of small size, owing to the pressure of wood 

 upon it. This is surrounded by a number of layers of wood, each ring 

 corresponding to one year's growth, and thus the age of the tree may be 

 determined. This wood is made up of cells, wood fibres, and vessels more 

 or less thickened internally. Surrounding the wood in winter is a thin 

 layer of thin-walled cells, termed the cambium (No. 8), to be considered 

 presently. Outside of the cambium is a ring or rind forming the bark, 

 now of considerable thickness by comparison with that of a Stock or 

 Gourd. It has lost its skin or epidermis (No. 1), and in place of the 

 stomata, openings loosely filled with cork cells, and known as lenticels, 

 may often be observed on stems or branches not too old. These are 

 breathing pores. A large portion of the bark is made up of corky tissue 

 (No. 2), gradually breaking away from year to year, while the inner 



