34 THE VEGETATIVE ORGANS 



compared. In the Maize, note the smooth pericarp, and 

 the remains of the stigma as a little point above the embryo 

 (Fig. 14, st), also the hard endosperm (e). A germinating 

 grain is shown in Fig. 12, 6. The Oat differs from the 

 Wheat and Mai2e in that the grain is covered with chaffy 

 scales. Other interesting examples belonging to the same 

 class are the Onion and the Wild Hyacinth. In these the 

 tip of the cotyledon remains in the endosperm and acts as 

 a sucking or absorbing organ (see Fig. 87, a, c, d). 



After seeds have ripened in the fruit they commonly 

 require to pass through a dormant period, which varies in 

 length in different species, before they are able to recom- 

 mence growth. So long as a seed is kept dry, or at a very 

 low temperature, growth does not take place, and some 

 seeds may lie dormant while retaining their vitality for very 

 many years ; but under the conditions we have provided 

 viz. moisture, air, and warmth germination begins. 



CHAPTER III 



STRUCTURE OF ROOTS 



Tissues of a mature dicotyledonous root. We have seen 

 how roots arise, also their form and mode of growth ; let us 

 next consider how roots are constructed. Fig. 15, 1 is 

 a photo-micrograph of a cross-section of an old root of a 

 Dicotyledon showing the different tissues of which it 

 is composed. The outer surface is composed of a layer 

 of cells called the epidermis (e) (Gr. epi = upon, derma 

 = skin). In the young root some of these cells grow out to 

 form root -hairs (see Fig. II, 7). Beneath the epidermis is 

 a wide ring of cellular tissue the cortex (Fig. 15, co) with 

 small air-spaces between the cells. The innermost layer of 



