52 Commercial Gardening 



7. MOVEMENTS OF WATER AND FOOD 

 PRODUCTS IN PLANTS 



Root Pressure. Having considered the mechanism and some of the 

 properties and contents of roots, stems, and leaves, the way is now clear 

 to discuss some of the phenomena exhibited by plants as a whole or in 

 a connected way. There is no regular circulation of sap in plants com- 

 parable to the blood in animals, nor a constant flow in any one direction, 

 except temporarily. The flow is in many and diverse directions, according 

 to the particular kind of work being conducted and the part of the plant 

 where it is taking place. The movement is a progressive, not a circulating 

 one. Root pressure, taken on the whole, is the most constant or continuous 

 force at work in causing a rise of watery fluid in the plant. The pressure 

 it exerts may be most readily observed in spring, when all the tissues of 

 deciduous trees and shrubs are gorged with water, prior to the expansion 

 of the leaves. The Vine exhibits this pressure in a marked degree, and 

 though it varies within limits, according to the size and vigour of the 

 plant, it has been found to support a weight of nearly 15 Ib. to the 

 square inch. This alone enables the sap to rise to the apex of the 

 longest rod, because the force of capillary attraction must also be reckoned 

 with when it is remembered that the cavities of the fibres and vessels of 

 the wood get filled with water and air a combination that is difficult to 

 move. Trees, shrubs, herbaceous plants, and annuals exhibit this root 

 pressure, but in low-growing plants it is most observable at night and 

 early in the morning, that is, some time after transpiration has ceased and 

 before it commences to take effect again with daylight and a drying atmos- 

 phere. Root pressure is of great importance to plants that are making 

 their growth, by keeping their tissues gorged and extended with water, 

 without which growth would be impossible. It is equally important to 

 deciduous trees and shrubs, which require a considerable force to expand 

 their winter buds and urge them into fresh growth. Plants grow more 

 rapidly by night than by day, because root pressure is then exerting its 

 full force. Even if this is not sufficient to raise water to the tops of the 

 tallest trees, a considerable pressure is exerted on the buds by the expansion 

 of the air bubbles in the water of the wood cavities as a result of the rise 

 of temperature in spring. 



Water of Transpiration. As above stated, this current is set up by 

 the action of the leaves, and by some has been described as a " transpiratory 

 pull". The effect it has towards the base of the stem is that of "suction". 

 It acts only during the day, and, in the case of deciduous trees, only comes 

 into play when the leaves are expanded and the air pores or stomata are 

 sufficiently developed to commence work. When transpiration has been 

 at work for a time the cavities of the wood fibres and vessels get drained 

 of their liquid contents and filled with air, root pressure is subjected to 

 a negative pressure as a result, and the up current of transpiration is 



