RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION, 939 



ippear each year at the same place. In some cases the whole plant is annually 

 renewed; all the parts which existed the previous year die off, and a complete 

 rejuvenescence of the individual is accomplished underground. In the Potato and 

 Artichoke only the apical parts of the underground lateral shoots swollen into tubers 

 remain over till the next year, the whole of the rest of the plant having perished. In 

 many of our native Orchids the rejuvenescence takes place in a similar way (see 

 p. 219 and fig. 158); and one of the most interesting cases of annual rejuvenes- 

 cence occurs in Colchicum auiumnale (see fig. 422). In these cases, with the excep- 

 tion of the Orchids, the reserve food materials accumulate in underground parts of 

 the axis; in other cases this takes place in the swollen roots, which remain in 

 connection with the underground part of the stem that bears the new buds, as in the 

 Hop, Dahlia, and Bryony. In bulbs again the reserve accumulates in the leaves 

 (bulb-scales) which surround the bud that developes into the new plant. The re- 

 serve often collects in cataphyllary leaves of peculiar development; in Allium Cepa 

 in the lower part of the leaf-sheaths, which persist through the winter, while the 

 upper parts of the leaves die off. 



We have already in the last chapter spoken of the immense variety of the 

 contrivances which have for their object the partial or entire prevention of the 

 self-fertilisation of plants, in order to produce a stronger and more numerous off- 

 spring by the sexual union of different individuals ; and only a few examples need 

 now be mentioned. Just as the form, size, colour, position and movements of the 

 parts of the flower are almost invariably adapted to facilitate the conveyance of 

 pollen from one flower to another, generally by insects, and often also to render 

 self-fertilisation impossible ; and as a great diversity even of those forms of flowers 

 which are constructed on the same morphological type results from this, so the 

 properties of ripe seeds and fruits are no less adapted^ to bring about the dis- 

 semination of the seeds. Fruits which are very similar from a morphological point of 

 view may nevertheless assume physiological properties which are altogether different, 

 and fruits which are very different morphologically may become extremely similar in 

 consequence of their adaptation to the purposes of dissemination. The service 

 rendered by insects in the fertilisation of diclinous, dichogamous, dimorphic, and 

 many other flowers, is performed by birds in the dissemination of a number of seeds 

 which are concealed beneath fleshy edible pericarps ; in some cases, as the Mistletoe, 

 it is scarcely possible to imagine any other mode of dissemination than the eating of 

 the berries by birds. Dry fruits or the seeds which are shed by dry fruits are often 

 provided with an apparatus adapted for transport by the wind, the morphological 

 value of which is as various as possible. The wings on the seeds of the species 

 of Abies are outgrowths of a superficial layer of the tissue of the scale, those on the 

 seed of Bignonia muricata originate from the integument of the ovule ; the wings of 

 the indehiscent fruits (samaras) oi Acer, Ulmus, &c. are outgrowths of the pericarp; 

 the crown of hairs on the seed of Asclepias syriaca evidently performs the same 

 service as the pappus of many Compositse which is a metamorphosed calyx. In 

 these cases it is obvious that the wind carries the seeds or fruits; in other cases 



* It is scarcely needful to mention again that this mode of expression has only a metaphorical 

 meaning from the stand-point here assumed, and is only used for the sake of convenience, 



