STRUCTURE OF MODIFIED SHOOTS 131 



' eyes '. Such swollen underground stems are known as 

 tubers. Cut a slice from a potato and test it with iodine 

 solution. What food-reserve is present ? make a watery 

 extract of a soft, sprouted potato and test with Fehling's 

 solution for grape-sugar. 



The Artichoke is another example of an underground 

 stem-tuber. The food-reserve here, however, is not starch, 

 as in the potato, but a substance allied to sugar, called 

 inulin. The above must not be confused with the root- 

 tubers of the Lesser Celandine and Dahlia. 



Corm of the Crocus. A short, thickened, underground 

 stem, similar in many respects to those we have considered, 

 is found in the Crocus (Fig. 84, 1). In this case, however, 

 the stem is somewhat globular and surrounded by mem- 

 braneous scale-leaves (sc) ; such a stem is called a corm. 

 Examine a dry corm ; determine the arrangement and 

 mode of attachment of the scale-leaves, and remove them 

 from below upwards. Examine a piece of scale with 

 a pocket lens and observe that the fibres which form its 

 skeleton are parallel, with many cross-connexions. Note 

 the difference in length of the internodes from the base of 

 the corm upwards, and the circular scale-scars (s.s). Are 

 there any axillary buds ? At the upper end of the corm, 

 where the scales are crowded together, two or more buds 

 arising in their axils become much larger than the rest. 

 Cut the corm vertically in two (see Fig. 84, 3). Note the 

 thick, solid axis with the veins (v) passing through it, and 

 try to trace one of these to a small axillary bud. Place 

 a drop of iodine solution on the cut surface. Of what does 

 the food-reserve material consist ? 



Dissect carefully one of the large upper buds, and com- 

 pare its parts with those of other buds you have examined. 

 Note, at the base, the membraneous scales followed by four 

 or five fleshy, cylindrical leaves or tunics. On removing 

 these, we find seven or eight small, pale-yellow foliage- 



1 2 



