The Science of Plant Growing 53 



dominant. When pot plants and those in the ground are well supplied 

 with water, and their leaves do not flag, it is clear that the great volume 

 of water being given off by the leaves must be coming directly from the 

 roots, and that the absorbent activity of the latter is equal to the demand 

 made upon them. It has been calculated that hundreds of pounds of water 

 are given off by the leaves of trees on a hot day (see p. 120). The water 

 of transpiration is the most rapid current of watery fluid known in plants, 

 and is most characteristic of woody plants; it is more feeble in herbaceous 

 plants with a less-developed vascular system; and non-existent in cellular 

 plants. Since the vessels of the wood are filled with air, the water must 

 necessarily travel in the walls of the wood, so that there is a continuous 

 passage or highway for it from the longest or remotest root fibres to the 

 tips of the leaves. In trees, like the Oak, Ash, Lime, Apple, Cherry, and 

 all other Dicotyledons, there is one main current through the trunk. This 

 flow is chiefly through the younger or sap wood, also known as alburnum, 

 less feebly through the heart wood or duramen, owing to obstructions 

 caused by age. The pith and bark may be removed without causing any 

 diminution in the rise of the sap. In Monocotyledons, like Palms, there is 

 not one main current, but hundreds of small ones in the trunk of a good- 

 sized tree. The transpiration current rises in the small, isolated fibro- 

 vascular bundles distributed through the ground tissue. 



In the case of the two great causes of the movements of water in 

 plants just discussed, it must be presumed that the temperature is adequate, 

 since they take place under natural conditions. When Vines, Peaches, 

 Figs, Melons, Cucumbers, and Tomatoes are being forced, out of their 

 natural season, the necessary temperature best suited to each subject 

 must be supplied artificially. Feeding, watering, and the amount of 

 atmospheric moisture have to be controlled likewise by the cultivator, if 

 the best produce is to be secured. Light is liable to be deficient in winter, 

 and full advantage must be taken of it, considering how vital it is to 

 growing plants, from the start till the fruit is matured. This requires 

 properly constructed houses with- the glass kept clean. Ventilation, during 

 the middle of the day, at least, is beneficial in promoting transpiration, 

 drying the atmosphere, keeping the functions of the leaves in healthy 

 condition, and hardening the tissues of stems and leaves by the proper 

 thickening of their cell walls, thereby preventing them from becoming 

 unduly " drawn ". The houses can be closed early in the afternoon, thereby 

 conserving the sun heat, if any, for that is more favourable to growth 

 than artificial heat. The foliage may be syringed, if the nature of the 

 weather for the time being warrants or requires it; transpiration will 

 be immediately checked and root pressure will soon begin to exercise its 

 powerful effect upon growth, so that no time is lost by giving timely 

 and judicious ventilation. 



Transport of Food Materials. All the food of green plants is manu- 

 factured in the leaves and other green parts, and it follows that it must be 

 transported or conveyed to the various points where growth is going on. 



