lo March, igi:;.] 



W/waf aJid its Cuhiva1'to)i. 



187 



This pnjcess is of eiinrmuus praclical importance in the economy of 

 nature. All animals directly or indirectly are dependent on the preliminary 

 life of plants to store up food, and green plants have the power of securing 

 the greater portion of the' s )lid matter of which thev are composed from 

 the carbonic acid gas of the air. Much discussion has taken place as t<> 

 exactly what happens in the process of assiniilation. Tn any case starch 

 may be regarded as the first visible product formed. Starch, however 

 does not accumulate in the leaf, but is transferred from the leaf to the 

 stem and roots to be stored till required. The starch is transferred bv the 

 agency of an enzyme — diastase — such as is formed in the germinating grain. 

 The starch is thus converted to sugar, which being soluble may diffuse to 

 such parts as require it. Photosynthesis takes place only in the day time, 

 whereas respiration is constantly in progress. Respiration, however, is 

 never so rapid as 

 the opi)Osite process 

 of assimilation so 

 that the net result 

 is an increase in 

 dry weight as a re- 

 sult of assimilation. 



Traiisfiratiou. — 

 A third function of 

 the leaf is to get rid 

 of the superfluous 

 water taken in by 

 the root-hairs. As 

 the mineral plant 

 food taken in hv 

 the ro<jt is in the 

 form of a very 

 dilute solution, it 

 follows that a large 

 amount of water 

 must be evaporated 

 in order that the 

 plant may recei\-e 

 sufficient mineral 



matter to build up "'' 

 its tissues. The 

 superfluous water is \aporized in the intercellular tissues of the mesophyll 

 and passes out through the stomata, whilst the mineral substances remain 

 behind. This important process is called transpiration and by its means 

 the absorption of fresh supplies of mineral food from the soil is rendered 

 possible. 



A large number of experiments have been made to determine the amount 

 of water lost by transpiration from the ordinary farm crops. Though 

 the actual amount is very considerable it is less than would be evaporated 

 from an equal surface of water exposed to the same conditions. Lawes 

 and Gilbert's experiments tend to show that for every ton of dry matter 

 elaborated in ordinary farm crops about 250 tons of water require to be 

 evaporated from the lea^■es. Hellriegel's observations in Germany show 

 that for wheat 453 tons of water are required for each ton of dry matter 



TRANSVERSE SECTION OF A STEM OF FEDERATION 

 WHEAT (X 50). 



