103 



93. The starch formed in the leaves 

 is not allowed to accumulate there, but is continually being 

 transported to other parts of the plant where it is required for use, 

 or storage. This transport goes on both day and night, but as, 

 during the day, the amount of starch manufactured is consider- 

 ably in excess of that removed, starch does accumulate in the 

 leaves to a certain extent, which , however, is usually removed 

 during the night. Thus leaves as a rule contain more starch at 

 nightfall than in the early morning. Only those substances which 

 are soluble are able to traverse the cell-walls and they can thus 

 be transported easily and quickly from cell to cell in a plant. 

 Starch, however, is not soluble in water at the ordinary tem- 

 perature, and hence, for the purposes of transport in the plant, it 

 is converted into a soluble sugar, such as glucose, or maltose, and 

 this is effected by means of the substances called enzymes. The 

 latter are chemical substances formed by the protoplasm 

 during metabolism which possess the power of more or less 

 altering various compounds without being themselves effected 

 and many are able to convert insoluble into soluble substances. 

 Diastase is a general term for a group of enzymes which are 

 frequently found in plants, some of which can turn starch 

 into glucose, or fruit-sugar, while others turn it into maltose, 

 or cane-sugar. In addition to this power of enabling valuable 

 food materials to be easily transferred from place to place, 

 enzymes are also frequently employed by plants for converting 

 compounds into substances which can be readily assimilated 

 and food materials stored in seeds are often thus made available 

 for assimilation by the young plant, 



94. All plants, like animals, K* s P iration - 

 require to breathe, and. like animals, plants absorb oxygen 

 and give off carbon dioxide, heat being evolved in the 

 process which is known as respiration. In the day time 

 respiration is, as a rule, not obvious in green plants growing 

 in the sunlight and which are found to continually enrich 

 the air with oxygen. This is due to the fact that the quantity 

 of oxygen evolved in carbon-assimilation is as a rule much 

 in excess of that required for respiration and that nearly all 

 the carbon dioxide produced during respiration is reassimilated 

 in the cells containing the green chlorophyll. If a green 

 plant, however, is placed in an air-tight jar and covered with 

 black cloth, or otherwise protected from the sunlight, it will 

 be found to absorb oxygen from the air in the jar and to give 

 off an equal volume of carbon dioxide. Again, if a quantity 

 of germinating seeds are placed in an air-tight jar and after 



