STRUCTURE OF MODIFIED SHOOTS 139 



leaves on the surface of the ground. Collect some of 

 the seedlings and study them carefully. On germination the 

 short radicle is carried downwards by the elongation of 

 the single cotyledon (Fig. 87, a and b). The tip of the 

 cotyledon is solid and remains in the seed, but it is tubular 

 below, and has a small slit (s) on one side. At the base of 

 the tube is the plumule, and when the first green leaf grows 

 it passes up the tube and out at the slit (c, d). The tip of 

 the cotyledon acts as a sucking organ and withdraws food 

 from the endosperm which is passed to the growing parts 

 below. The narrow green leaf is the first organ of photo- 

 synthesis. The food thus obtained accumulates in the 

 bases of the cotyledon and foliage-leaf, and in consequence 

 they become swollen and form a small bulb (Fig. 87, d, e). 

 As new leaves are formed on the stem, the bulb increases 

 in size and begins to'descend farther into the soil. It does 

 this by the elongation of its base (f to l), and soon 

 a curiously elongated bulb results. Often you will find 

 on these bulbs large contractile roots (f, g, h, i, l), which 

 aid in descent. As soon as the work of contraction is 

 completed, a separation-layer forms across the base of each 

 root, which then decays (Fig. 87, 1) and leaves a root-scar 

 on the bulb (f, l, sc. and cr.sc). 



If several elongated bulbs are planted, allowed to grow 

 for a few weeks, and examined at intervals, you will find 

 that the long, outer, fleshy scale-leaves give up their food- 

 reserve and decay (h, i). New green leaves and colourless 

 scales are formed on the short stem within ; these in turn 

 become swollen at the base and form an oval bulb (j). 

 At the base of each bulb roots of two kinds are formed : 

 (1) slender, fibrous roots, and (2) long, thick roots which 

 eventually become contractile. The processes of elongation 

 of the bulbs and the formation and shortening of the con- 

 tractile roots are repeated each season until the requisite 

 depth is reached ; then both processes cease. At each 



