6 SCIENCE AND FRUIT GROWING 



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off new phloem on the outer side, and new xylem on the inner, 

 the vascular tissue becomes concentric, instead of being in groups. 

 The older wood is towards the centre, and, in trees which live 

 for many years, this gradually ceases functioning, forming what 

 is known as heart wood : the younger wood nearer the cambium 

 carries on the necessary activities, and forms the sap wood. The 

 epidermis cannot stretch sufficiently to keep up, with the growth 

 in thickness of the stem, so a fresh protective coat is formed, 

 known as bark : a cork cambium is developed from cells towards 

 the outer limit of the cortex, and the cells actively divide, forming 

 a layer of cork, with rectangular cells arranged very regularly. 

 As growth proceeds, and the pressure from within becomes greater, 

 this cork is ruptured, and a new cork cambium is formed further 

 in. The layer of cork, with the broken and dried-up tissues 

 outside, forms the bark. Breathing spaces are provided by means 

 of lenticels in the bark (see Fig. 2) small openings in the 

 regularly arranged cork, filled with spongy cork, made up of 

 round cells, loosely packed, and permitting the passage of air. 

 In the winter, when the physiological functions of the plant are 

 largely in abeyance, these lenticels are blocked up by a layer of 

 regular cork, which is broken down again as soon as active growth 

 is resumed in the spring. 



When an old stem is cut across, the wood is seen to be 

 arranged in concentric rings, due to a difference in the growth 

 of the new wood in spring and autumn. In the first flush of the 

 year, when the plant is growing very actively, there is need for an 

 abundant supply of water from the roots, and the wood vessels 

 formed at that time have large cavities to meet this need : as 

 time goes on, and the rate of growth slows off, less water is needed, 

 and the autumn wood has, consequently, only small cavities. The 

 alternation of large and small wood vessels results in the con- 

 centric rings which are called annual rings, and these enable the 

 approximate age of a tree to be estimated. 



LEAF. The leaf may be regarded as the main laboratory of 

 the plant, as it is so intimately associated with the physiological 

 processes of nutrition, of breathing, and of the excretion of water. 

 The essential part of a leaf is its broad flat blade, which is bounded 

 on its upper and lower surfaces by an epidermis (Fig. 3). Be- 

 tween these are a few layers of cells, of which the upper rows are 

 often arranged more or less regularly, forming the palisade 

 tissue, while the lower cells are arranged very irregularly, with 

 large intercellular spaces, forming the spongy mesophyll. Vas- 

 cular bundlesjbranch off from those in the'stem, and, passing up 



