INTRODUCTION. 



35 



Fig. 32. 



summer, as at these periods, the requisites for this process are all afforded to 

 the fullest extent; this is also favoured by covering them loosely with soil, 

 so that whilst the light is excluded, they may experience the vivifying influ- 

 ence of the sun's rays, and at the same time be kept in a moist state. When 

 planted at too great a depth, they remain torpid, from not receiving the sti- 

 mulus of air; and when they have not a proper covering of earth, they do 

 not germinate, from not obtaining sufficient moisture. 



When germination commences, the moisture absorbed 

 softens all parts of the seed, a chemical change takes 

 place in the starch of the embryo or cotyledons ; the 

 embryo enlarges and bursts the integuments ; the radicle 

 protrudes and descends, often attaining some length before 

 the plumule appears ; the albumen is gradually absorbed ; 

 the cotyledons expand and become seminal leaves, which 

 afford nourishment to the young plant, by elaborating sap, 

 and wither when the true leaves appear or remain under 

 the surface, and are gradually absorbed and disappear. As 

 soon as the root and leaves are developed, in opposite 

 directions, the process of germination is perfected, and 

 the new plant is formed. 



Propagation by buds. This takes place in several ways : 

 by means of the bulbs which grow at the base of the scales 

 in what is called a bulbous root; these bulbs are soon de- 

 tached and become independent plants; by means of bulb- 

 lets growing in the axil of leaves, as in some species of lily, 

 or in the place of flowers, as in the tree onion ; by means of 

 buds growing at the margins of leaves, as in Bryophyllum 

 (fig. 15) ; by means of buds or eyes in the subterranean 

 stems of plants, as in the potato. All these resemble seeds 

 in producing new plants, when placed in proper situations, 

 but differ from them in not being the product of fecundation, but being exten- 

 sions of the parent plant. 



Propagation by layers or slips. The first of these is accomplished by 

 surrounding a branch with moist earth, in which case it often throws out roots 

 and becomes an independent plant, more especially if the bark be wounded, 

 or a ligature be passed round it at the part placed in the earth. Sometimes 

 this process takes place naturally, the branches bending over so as to touch 

 the ground, and then throwing out roots at their point of contact, as in the 

 Raspberry. Propagation by runners is somewhat analogous. That by slips 

 is essentially the same as the foregoing, the only difference being that the 

 branch or slip is wholly detached from the parent stem. There is one fact 

 connected with this mode of propagation deserving of notice, that the number 

 of seeds in the fruit produced by the new plant, is always less than in one 

 arising from seed. Propagation by grafting and inoculating is somewhat 

 analogous. This operation consists in uniting a part of one plant to another ; 

 this union is effected by the cambium, and hence it is requisite that the barks 

 of the graft and stalk should be placed in apposition. The cambium exudes 

 from each; becomes organized and. produces a union. The operation of 

 inoculating, or budding, is essentially the same, but in this case a bud and a 

 slip of the environing bark only are used, and not a slip or cutting, as in 

 grafting. In both cases they only succeed when the stalk, and the cutting or 

 bud belong to the same natural family. A graft is merely an extension of 

 the parent stem from which the scion was derived, and not a new individual. 



Plumule, b Radicle, 

 c Cotyledons. 



