620 PHYSIOLOGY. 



be fully explained by our present knowledge, but doubtless depend on 

 causes similar to those just adverted to ; among these are the facts that the 

 Orange succeeds better on a Lemon-stock than on one of its own species, 

 while the Apricot does better on its own species than on the Plum, &c. 

 The influence of the physiological conditions of the stock upon the scion 

 is turned to account by gardeners in producing a dwarfer " habit " and 

 an earlier and more profuse production of fruit. Thus Apples grafted on 

 the low-growing " Paradise stock " assume the dwarf habit of the stock 

 and become more prolific. So Pears on the Quince-stock not only are 

 dwarfed in size, but produce fruit much more abundantly than when 

 grown on their own roots or grafted on another kind of Pear. Gardeners 

 often practise " ennobling" fruit-trees, where buds and grafts are attached 

 upon stocks of good varieties of the plant in preference to wild stocks. 

 Thus Apples are said to be much superior when grafted on stocks of good 

 varieties instead of on Crabs, &c. ; and a kind of crossing of the qualities 

 of varieties has been attempted on this principle, grafting kinds which 

 bear sickly-flavoured Apples upon stocks of rougher varieties, Jargonelle 

 and " mellow " Pears upon later, gritty varieties, &c. " Double graft- 

 ing " is done when it is desired to secure a particular kind of fruit which 

 will not unite or graft with the ordinary stock ; thus a Pear may be 

 grafted on a Quince-stock, and on the scion may be grafted another Pear, 

 which will not unite directly with the Quince. Further details on the 

 subject of grafting, a most important and interesting one, must be sought 

 in horticultural works. 



Sect. 2. SEXUAL KEPEODTTCTION. 



Preliminary Observations. In almost all plants the greater 

 part of the active existence is passed in the development of vege- 

 tative organs, increasing the bulk of the individual, or occasionally 

 also accompanied by multiplication of the plant by mere subdivision 

 into parts. But at certain epochs another tendency manifests 

 itself : the energies of the plant become concentrated in the forma- 

 tion of what are called reproductive organs, for the purpose of 

 producing and maturing those independent germs of new indi- 

 viduals of the species called spores and seeds. 



The formation of reproductive structures bears a very interesting 

 relation to the vegetative development. Generally speaking, the repro- 

 ductive organs are only formed when the vegetative structures have 

 become healthily developed, so as to accumulate a certain amount of 

 assimilated matter in the substance of the plant. We observe that many 

 garden plants grown in unfavourable soil, in shady localities, &c. will not 

 flower; and the number of years that elapse before the flowering of such 

 plants as the Agave, Talipot Palm, &c., varies with the more or less 

 favourable climate and soil ; moreover, in ordinary cases, the flowering 

 takes place at the close of the season of growth (except where the flowers 

 emerge from buds provided for by the previous year's vegetation, as in 

 Apples &c., in biennial and many perennial herbs). This indicates that 

 vigour of the vegetative organs is a necessary condition of reproduction. 



