HOW PLANTS OBTAIN FOOD. 



9' 



excreted by the cells of the gland, which glistening in the sun- 

 light reminds one of drops of dew. For this reason the plant 

 is called the sundew. When an insect alights on a leaf the 

 viscid substance clings to it and holds it firmly so that it 

 cannot escape. The glandular hairs then begin slowly to curve 

 inward toward the centre of the 'leaf as shown in figure 68. 

 Finally the margins of the leaf become inrolled also, so that 

 the insect is held fast and close to the upper surface of the 

 leaf. , Excretions from the leaf surface act as a digestive 

 ferment upon the insect. 



162. Nutrition of bacteria. — Bacteria are very minute plants, 

 in the form, of short rods, which are either straight or spiral, 

 while some are minute 

 spheres. They are widely 

 distributed; some cause dis- 

 eases of plants- and animals, 

 others cause decay of organic 

 matter, while still others play 

 an important role in con- 

 verting certain nitrogen com- 

 pounds into an available form 

 for plant food. They absorb 

 their food through, the sur- 

 face of their body. They 

 may be obtained in abund- ^ h ™ d n *3d 

 ance for study in infusions lobes ' 

 of plants or of meats. 



To demonstrate bacteria in infusions take a small quantity 

 of hay or of meat. Place it in water and heat at about 60° C. 

 for an hour. Then set the vessel containing the infusion aside 

 in a warm room for several days., Numbers of bacteria will be 

 developed, some of them probably motile. With a good micro- 

 scope they may be demonstrated by mounting a drop of the 

 infusion on a glass slip and preparingfor examination with the 

 microscope. 



Fig. 67. 



Leaf of Venus fly- 

 trap (Dionaea musci- 



Fig. 68. 



Leaf of Drosera ro- 

 tundifolia, some of the 

 glandular hairs fold- 

 ing inward as a result 

 of a stimulus. 



