160 STRUCTURAL BOTANY 



The base of the stem, which bears the scale-leaves 

 of the bulb, has a rather different structure. The 

 distinction between central cylinder and cortex is less 

 sharp ; there are no chlorophyll granules in the cortical 

 cells, because this part of the stem is never exposed 

 to light. The vascular bundles have a different course, 

 as explained above. The ground - tissue contains 

 quantities of starch and also oil. The reserve 

 substances of the bulb stem are, however, used up in 

 summer, when the flowering stem develops, and then 

 we find its cells almost empty. The bulb - scales, 

 which we shall describe later on, are more important 

 than the stem as storehouses of food. 



b. Leaf 



It matters little whether we take an autumnal 

 ground-leaf or a summer leaf from a flowering stem. 

 They are different, as we have seen, in shape, but in 

 internal structure they are very much alike. 



We have already learnt the general course of the 

 veins. In the midrib and principal veins of the larger 

 leaves several vascular bundles run side by side ; the 

 smaller veins correspond each to a single bundle. If 

 we trace the bundles outward from the stem into the 

 leaf, we find that they immediately begin to branch, 

 the main branches taking a parallel course, while they 

 are connected by minor oblique strands (see Fig. 

 62, v.1>). 



Thus the whole surface of the lamina is provided 

 with an elaborate system of conducting channels, 

 chiefly differing from that of our dicotyledonous type 



