RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 939 
appear 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 (samarse) of 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 Compositae 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. 
