116 FOUNDATIONS OF BOTANY 



strength, but only those which are well adapted to store 

 the requisite amount of food. 



125, Structure of a Bulb ; the Onion. Examine the external 

 appearance of the onion and observe the thin membranaceous skin 

 which covers it. This skin consists of the broad sheathing bases of 

 the outer leaves which grew on the onion plant during the summer. 

 Remove these and notice the thick scales (also formed from bases 

 of leaves as shown in Fig. 48) which make up the substance of the 

 bulb. 



Make a transverse section of the onion at about the middle and 

 sketch the rings of which it is composed. Cut a thin section from 

 the interior of the bulb, examine with a moderate power of the 

 microscope, and note the thin-walled cells of which it is composed. 



Split another onion from top to bottom and try to find : 



(a) The plate or broad flattened stem inside at the base (Fig. 47). 



(5) The central bud. 



(c) The bulb-scales. 



(d) In some onions (particularly large, irregular ones) the bulblets 

 or side buds arising in the axes of the scales near the base. 



Test the cut surface for starch. 



126. Sugar in the Onion. G-rape sugar is an important 

 substance among those stored for food by the plant. It 

 received its name from the fact that it was formerly 

 obtained for chemical examination from grapes. Old 

 dry raisins usually show little masses of whitish material 

 scattered over the skin which are nearly pure grape sugar. 

 Commercially it is now manufactured on an enormous 

 scale from starch by boiling with diluted sulphuric acid. 

 In the plant it is made from starch by processes as yet 

 imperfectly understood, and another sugar, called maltose, 

 is made from starch in the seed during germination. 



Both grape sugar and maltose (and hardly any other 

 substances) have the power of producing a yellow or 



