CHAPTER III. 



THE ROOT. 

 MORPHOLOGY. 



A sunflower or bean-plant may be regarded as consisting of 

 two slender cones placed base to base (fig. 2). One of these, the 

 primary stem, grows upwards ; the other, the primary root, down- 

 wards. When such a main axis forms the most prominent part 

 of the root, it is said to be a tap-root, and this is very strikingly 

 seen in the carrot, turnip, and radish. The primary root 

 generally possesses numerous branches, and these may be re- 

 garded as so many slender cones attached by their bases to the 

 main one. These secondary roots are commonly in the form of 

 fibres. But the primary root is very often extremely short, and 

 in that case {e.g., grasses) the plant is fixed in the ground by 

 means of adventitious roots (fig. 4), which usually grow from the 

 stem, but may also arise from the leaf-stalks or leaves. Such 

 roots may also be present in addition to the primary and secondary 

 ones, as, for example, in the ivy, where they are used for climbing. 

 The " striking " of cuttings means the development of adventi- 

 tious roots from the end of a piece of stem pushed into the soil 

 The majority of roots are underground, but aquatic plants possess 

 icater-roots, and air-roots are also known. These last may be the 

 only ones present, as in many tropical orchids ; or, as in the ivy, 

 they may exist in addition to roots of the ordinary kind. The 

 orchids in question are epiphytes, that is to say, they simply live 

 on other plants, but not at their expense. Plants exist, however, 

 such as the clover- dodder, and mistletoe, in which the roots are 

 parasitic, penetrating and deriving nourishment from the tissues 

 of other plants. 



Young roots are white or pale in colour, old ones generally 

 brown. Under no circumstances is the green colouring matter, 

 chlorophyll, present. Leaves are never found upon the root. 



The external form of a root system depends mainly on three 

 things : (1) the presence or absence of a tap-root; (2) the nature 



