32 THE VEGETATIVE ORGANS 



A section of the embryo is shown in Fig. 13. Below is the short 

 radicle (ra) ; above is the straight plumule (pi). On one 

 side is a shield-shaped structure, the cotyledon or scutel- 

 lum (sc), with its convex face applied to the starchy food- 

 reserve ; and opposite the point of attachment of the shield 

 is a tiny scale, the ligule (/). 



The Wheat grain thus consists of a fruit-coat and seed- 

 coat fused together, an embryo consisting of a radicle, 

 a plumule, a single cotyledon (the scutellum), and a 

 ligule ; and, occupying the larger part of the grain, is the 

 food-reserve, which is outside the embryo and therefore 

 called endosperm (e). (Compare this with the fruit of the 

 Ash, Fig. 9.) 



Plants which contain only one cotyledon in the embryo 

 are placed in a large class called Monocotyledons. 



Germination of Wheat and Maize. Examine germinating 

 grains in different stages of growth, and note the behaviour 

 of the parts of the embryo (Fig. 12, 3, 4, and 5). As the 

 radicle (ra) elongates, it bursts through the thick tissue 

 which surrounds it, and now encircles its base as a root- 

 collar (r.c). Continuing to grow, it becomes clothed with deli- 

 cate root-hairs (r.h) everywhere except at the tip. Look 

 for the root-branches (a.r) and determine their point of 

 origin. Note that some spring from the base of the stem. 

 Roots arising from the stem are called adventitious roots 

 (Fig. 12, 3 and 4 a.r). These also bear root-hairs. Note 

 how the particles of fibre adhere to them and are not easily 

 washed off. In Wheat the radicle does not become the main 

 root : the fibrous roots of the mature plant all arise from 

 the nodes of the stem. 



Compare the mode of emergence of the plumule (pi) with 

 that of the Bean and other seedlings examined, and note 

 the differences. In the Wheat the leaves are rolled up and 

 enclosed in a smooth, colourless, tubular leaf, the whole 

 forming a compact structure well adapted for boring its 



