PLAXT LIFE 17 



of trees, shrubs and annual plants, in the stem or shoot of which 

 there is a soft central cylinder of pith, surrounded completely by 

 wood, and external to this by cortex and then bark. The bark or 

 cortex is separated from the wood by the fibrous layer or bast, on 

 the inner side of which is a thin, watery or viscid layer called the 

 cambium or formative tissue. In plants of this type which exist 

 for more than two years, new layers of wood and bark are succes- 

 sively formed from the cambium. In the second type of sh-ii 

 (monocotyledon), as occurs in palms, bamboos and grasses, the 

 pith occupies the whole of the stem, the wood and bast being 

 scattered throughout in strands (vascular bundle*), instead of being 

 continuous and solid ; there is no separable bark, that being 

 represented by a hard external layer called the rind ; in these the 

 stem is hardest externally (whilst in the former type the hardest 

 parts of the stem are internal), there is no cambium, and therefore 

 the stem does not normallv increase in thickness. 



The leaves are usually composed of a thin layer of green 

 tissue termed the mesophylL which contains the chlorophyll bodies 

 or green colouring matter. This is held together by a framework 

 of ribs commonly known as veins, or botanically as vascular bundles. 

 Each surface is protected by a special layer of cells known as the 

 epidermis. In the lower surface of ordinary leaves are microscopic 

 openings, called stomala, leading into the substance of the leaf ; 

 through these the inhalation and expulsion of air and moisture take 

 place. In a few exceptions the leaves are placed edgewise towards 

 the source of light, as those of Eucalyptus, Loranthus, etc., the 

 stomata in these being equally distributed over both surfaces of the 

 leaf ; in floating leaves, as in Water-lily, the stomata are on the 

 upper surface. The functions of leaves may be compared to those 

 of the stomach and lungs of animals. 



The roots, stems and leaves being concerned in the supply and 

 elaboration of food, are called Organs of Xutrition, whilst the 

 Mowers are the Organs of Reproduction, producing fruits and 

 seeds. 



Flowers. A typical Mower consists of four distinct series or 

 circles of parts, the outer of which, composed of small green, leaf- 

 like organs called sepals, forms the calyx ; the next inwards, a circle 

 of brightly coloured and sometimes scented leaves, called petals, 

 forms the corolla; when both sepals and petals are similar in 

 appearance and colour, as in Lilies, they are known collectively as 

 the perianth; this term is also sometimes used when one of the 



