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JOUENAL OF THE KOYAL HORTICULTURAL SOCIETY. 



speaking generally, be paralleled one with another, allowing for the total 

 arrest of one or more elements in certain cases. Stems can form roots 

 when placed in the same conditions in which roots are formed, viz. dark- 

 ness and moisture. Aerial roots can take on the functions of stems ; and 

 roots formed underground can become stems and branches when exposed 

 to air and light. 



Now the fact I wish to emphasise is, that these differences between 

 stems and roots are, as a rule, the direct results of the aerial and subter- 

 ranean conditions of their normal existence respectively, and that when 

 a subterranean root becomes superficial or aerial it at once tends to 

 approximate to the character of a stem ; and if a normally aerial stem is 

 made to grow underground, it in turn tends to acquire the characters of 

 a true root. Not only does Nature often supply us with illustrations of 

 these changes, but experiments distinctly verify this assertion. 



When the roots of Elms, Poplars, Hawthorns, Horse-chestnuts, &c., 

 get exposed on the sides of banks, they constantly send out a perfect forest 

 of leafy shoots. 



The following experiment, recorded by Dr. Lindley, will also illustrate 

 this. A young AVillow-tree had its crown bent down to the ground ; this 

 was covered with earth, and soon emitted an abundance of roots. The 

 tree-roots were then carefully removed from the soil and the stem 

 inverted. The roots now became branches and emitted buds, and the 

 tree grew ever afterwards upside down. 



Judging by these experiences, it would seem that when roots are near 

 the surface, or exposed to light, these conditions appear to act as causes or 

 stimulants to the production of leaf-buds. 



But if the aerial portions of a tree — say an Elm — be cut down, then 

 the vitality of the roots asserts itself in the production of buds from the 

 roots ; such not having been previously exposed nor buds formed. 



This property can be taken advantage of ; so that j)lants can be 

 propagated by pieces of the roots, as of Peaches, Plums, &c., which 

 readily give rise to buds. 



In fact, in the last named of the genus Prunus, it has become a fixed 

 habit, the production of shoots from the roots being particularly abundant 

 and troublesome in a garden. Similarly, Raspberries throw up numerous 

 stems ; and these will be found to issue from the horizontally spreading 

 roots. The new shoots arise from the internal layer of tissue of the root, 

 called the "pericycle," and issue by dissolving the superincumbent 

 cortical layers. Hence they are said to be " endogenous " in their 

 growth, i,e. issuing "from within." 



Subterranean Stems. — Various modifications of these exist for propa- 

 gative purposes. If it be a more or less globular body, it is called a corm, 

 as of Crocus, Gladiolus, Cyclamen, &c. The function is to store up a 

 quantity of nutriment for the bud upon it, when it begins to grow into 

 an aerial plant. If several thick internodes form a horizontal mass, it is 

 called a rhizome ; if vertical, a root-stock, as of a Primrose. If under- 

 ground branches cease to elongate, their last formed internodes swell into 

 tubers, as of the Potato. 



If underground shoots can elongate freely they form creeping stems, 

 as in Grasses and Sedges on sand-hills by the sea. 



