58 THE VEGETATIVE ORGANS 



forms so large a proportion of its substance ? But this 

 is a question we cannot answer until we come to the work 

 of leaves. 



From the above observations we learn that in many 

 plants roots arise from the radicle which grows downwards 

 as the tap-root, e.g. Bean, Pea, Oak. In others, e.g. Wheat 

 and most Grasses, the radicle soon dies and is replaced by 

 adventitious roots from the stem. Roots fix the plant in 

 the soil and absorb weak solutions of mineral salts ; the 

 absorbing area being increased by branching and to a 

 greater extent by root-hairs. Usually, roots contain no 

 chlorophyll and bear no leaves, therefore the branches are 

 not axillary. In a young root, the groups of wood and 

 bast alternate with each other, and the first -formed wood 

 develops towards the centre (i.e. centripetally). From cells 

 of the pericycle and opposite the groups of primary wood, 

 branch roots arise in vertical rows. The tip of each root 

 is protected by a root-cap and possesses a sensory region 

 which is able to perceive a stimulus and transmit an impulse 

 to the neighbouring tissue, where growth occurs. The root 

 is sensitive only in the presence of oxygen. The direction 

 of growth is determined by the nature of the stimulus, 

 i.e. towards the soil, water, and food; and away from 

 obstacles and light. Some roots, swollen with a large 

 cellular tissue, store much starch, sugar, inulin, and other 

 reserves of food. Some of these we will now consider. 



CHAPTER V 



FORMS OF ROOTS 



We have seen that in Dicotyledons (e. g. Bean, Pea, and 

 Stock) the radicle of the embryo grows downwards and 

 becomes the primary root of the plant. From the tap-root, 

 branches or secondary roots arise which in turn give off 



