836 ORIGIN OF SPECIES. 



very survival of the plant in the struggle for existence. The terms Purpose, Adapt- 

 ation, and Metamorphosis express therefore the same thing, and may be used as 

 synonymous, as we have already repeatedly done. 



For the purpose of the questions to be treated of in the following paragraphs it 

 is important to have as clear a conception as possible of the relationship of adapt- 

 ation to the morphological nature of the organs, and of the great constancy of 

 morphological characters and the infinite diversity of metamorphosis ; for such re- 

 lationship can be explained by no other theory than that of descent. 



In its most general features the relationship of adaptation to the morphological 

 nature of organs is manifested in the fact that all the various morphological members 

 perform the most different functions and in an infinite variety of ways ; in other words, 

 that the morphological nature of the parts of a plant is not directly determined by 

 their function, nor is the function of an organ determined directly by its morpho- 

 loo-ical nature. Thus, for example, trichomes sometimes take the form of a pro- 

 tective envelope (mostly in buds), sometimes of glands, sometimes of absorptive 

 organs (as root-hairs), sometimes of asexual organs of reproduction (as the sporangia 

 of Ferns), &c. The leaves again are usually organs of assimilation containing chlo- 

 rophyll ; but they may also be employed as protective envelopes to winter-buds (in 

 most of our native woody plants), as reservoirs for reserve food-materials (in the 

 seedlings of flowering plants and in bulbs) ; in Vascular Cryptogams they bear 

 the sporangia. In flowering plants the organs of reproduction and their envelopes 

 are peculiarly metamorphosed leaves ; in many slender-stemmed Angiosperms the 

 leaves are transformed into tendrils, in order to raise up the slender stem and fix it 

 to neighbouring supports ; the leaves of Nepenthes produce at their apex an append- 

 age which forms a pitcher provided with a moveable lid and filled with the fluid 

 which it itself secretes ; some of the leaves contained in the flowers are developed 

 into nectaries and then perform the function of glands ; not unfrequently they are 

 transformed into hard woody spines ; in other cases they are sensitive to irritation, 

 contractile, and so forth. The parts of the axis are scarcely less varied in their 

 development ; sometimes they cling round upright supports ; sometimes they are 

 woody and able to retain themselves in an erect position ; sometimes they are slender 

 swaying branches, or thick fleshy succulent masses (Cactus), or round tubers filled 

 with food-materials (Arum, potato), or they become tendrils (the vine), or spines 

 (Gleditschia) ; sometimes they assume the form of foliage-leaves (Ruscus, Xylo- 

 phyllum, &c.). The adaptations of roots are less numerous ; usually filiform, slender, 

 cylindrical, and provided with root-hairs for absorbing water and dissolved mineral 

 substances, they become tuberous reservoirs for reserve food-materials in the dahlia ; 

 their tissue is loose and contains air and they resemble swimming-bladders in 

 jussiaea ; in the ivy, Ficus repens, &c., they are simple organs of attachment for the 

 stem ; in Vanilla aromatica they play the part of tendrils ; but they never produce 

 sporangia or sexual organs. 



According to the definition already given of Purpose in the vegetable organ- 

 isation, its relationship to the morphological nature of the organ can also be illus- 

 trated by keeping in view the purpose to be served, i. e. the character of the 

 plant which is serviceable in the struggle for existence, and then observing the 

 means employed for attaining this purpose, /. e. what members of the plant are 



