THE HIGHER DICOTYLEDONS. 361 



traced back to the lower part of the stem and arise from it 

 in the axils of small leaves. In seedlings it is easy to make 

 out that the tuber-bearing branches come off above the coty- 

 ledons, so that the tubers are not roots, being developed 

 above the root-system. Why are potato-fields ridged with 

 the plants occupying the ridges ? Why is the soil heaped 

 up around the plants ? What happens if you do not 

 " ridge " a Potato-plant in the garden ? Have you ever seen 

 tubers growing in the air, in the axils of foliage-leaves ? 

 What happens if you cut off the underground tuber-bearing 

 branches ? 



Cut across a tuber, and test for starch with iodine solution. Put a 

 drop of water on a glass slide and dip a cut piece of tuber into the 

 water : notice the small white starch-grains which escape from the 

 opened-up cells of the tuber and become suspended in the water. If 

 you have a microscope, examine the grains, some of which will show 

 delicate lines corresponding to thin layers built up around the first- 

 formed portion of the grain (Fig. 148). Do the lines run evenly around 

 the centre of the grain, or are they closer together at one end ? 



These starch -grains are formed from sugar by the activity of small 

 colourless protoplasmic bodies called leucoplasts, distinguished from 

 the chloroplasts found in green parts by containing no chlorophyll and 

 by being unable to build up sugar and starch from carbonic acid. 

 Compare the results given with the iodine test in very young and 

 small tubers with the starch -con tent of larger tubers. Compare the 

 taste of (1) very young tubers, (2) fully developed tubers, (3) tubers 

 which have sprouted and given out well-grown shoots. Test tubers in 

 these different stages for sugar with Fehling's solution. Does the 

 tuber contain any other kind of reserve-food besides starch ? Shave 

 off a very thin slice from the outer portion of a tuber and test it with 

 iodine : the outer layer contains proteids. Note the layer of cork 

 which forms a skin over the tuber ; if this is shaved off, a new layer is 

 formed. What is the use of the corky skin ? 



Note the arrangement of the leaves on the stem of the 

 Potato-plant. Are the leaves simple or compound, with 

 stipules or without ? What peculiarity do you notice in the 

 sizes of the leaflets in each leaf? The branching of the 

 shoot, especially in the upper part, is rather complex ; in 

 most plants of the Potato family the branches and inflores- 

 cences do not arise strictly in the leaf -axils. 



Examine the flowers (Fig. 149), arranged in more or less 

 " flat-topped " inflorescences ; the short calyx-tube with five 

 sepals ; the five-lobed funnel-like corolla ; the five stamens 



