120 



of water necessary to add one pound to the weight of the dry matter in 

 a plant. It will therefore be readily understood that a great concen- 

 tration must take place before the dissolve! matter can be utlised. This 

 concentration takes place in the leaves. Drops of pure water ooz3 

 through the tiny pares, or stomata, on the leaves and are then evapo- 

 rated. The stomata are so made that they can regalate the a nount of 

 water that is transpired. When there is a liberal supply of water pass- 

 ing up to the leaves the two * guard cells/ which by reason of their 

 oncave shape form the opening, become turgid. Tais causes them ta 

 become still more concave, thus increasing the size of the opening. 

 IMore water is in consequence able to piss out If on the other hand 

 the plant is becoming dry, these cells collapse closina^ the opening. 

 The amount of water transpired is thereby very much diminished. 

 This function is such an important one that there is an intimate con- 

 nection between the form of the leaf and and the amount of transpira- 

 tion required. 



Take examples of plants grovn under very different coiditions — the 

 banana growing in moist situations, requiring a large supply of water, 

 and the cactus plants which grow in dry sandy places ind seem to re- 

 quire no water at all. What is the difference in the form of the leaf ? 

 The huge leaf of the banana increases the number of the sto nata to 

 such an extent that the plant is able to get rid of an enormous amount 

 of water, while the leaf of the catcus has almost entirely disappeared 

 in order that the number of the s omata may be so reduced as to check 

 transpiration. 



A simple experiment will prove that this process takes place. 

 (Jover two tumblers partially filled with water with post carls t'arough 

 each of which a small hole has been bored. Through the hole in each 

 card insert a fresi green twig, seeing that it fits tightlv' and dips into 

 1 he water. Cover each twig with another tumbler, clean anl dry. 

 One pair of tumblers should now be placed in the sunlight and the 

 other in a dark corner. If these are examined after a few minutes, it 

 Nvill be found that the upper tumblers are dim with moisture . 

 This dimness will be apparent to a ver}^ much greater extent in the 

 case of the one that has been exposed to the sunlight. This experi- 

 ment proves not ouly that plants transpire, or give off water vapour, 

 but also^that this action takes place much more quickly in the sunlight. 



If it were not thatp'ants lose water by transpiration, cuttings, bud- 

 wood, &c., might be kept for any lenojth of time without withering. 

 It is owing to this work of the leaves that it is found necessary to di- 

 vest the budwood of all its leives before proceeding to cut out buds, 

 in order that its moisture mny be preserved. 



Normal transpiration is beneficial to the plant because it helps to 

 cause the upivard flow of the sap As soon as it becomes excessive, 

 however, as in times of drought, the plant begins to suffer. As soon 

 as the amount of water transpired is in excess of what passes in at the 

 roots, the leaves will wither. 



The amount of water contained in a plant varies considerably with 

 the kind of plant, and, in the same plant, with the period of growth. 

 Further, some parts of the same plant contain more water than 

 other parts, e.g. the average amount of water in a plant is from 60 to 

 80 per cent, but the percentage in a ripe fruit is considerably hig .er. 



