Cytology

The primary wall, laid down in the manner above described, might be regarded as common to the two cells concerned in its formation, but many facts go to show that it is either a double membrane from its inception or that it readily splits into two. The result is that every cell is surrounded by its own individual wall. This is seen very clearly in the development of elongated cells such as fibres, which may grqw to many hundred times their original length. In this process they may change their position, sliding over the surface of neighbouring cells and making contact with cells of different, and of more remote, origin. Such elongated cells as the fibres undergo considerable thickening of their walls when their growth is completed. This thickening is originally of cellulose, but it may undergo secondary changes becoming wholly or in part lignified, thus rendering the wall firm and hard and better adapted to serve the purpose of mechanical support. Thickening and lignification of the wall is also characteristic of the cells concerned in the formation of the water-conducting elements. With the completion of these changes in the cells of the strengthening and water conducting tissues the protoplasm disap pears and the walls alone remain.

Plasmodesma.

The living cell in a complex multicellular plant body is a unit, but not an independent one, and its functions must be conditioned by, and related to, the activities of the organ ism as a whole. This must involve a close connection between the living protoplasm of neighbouring cells and this is effected by deli cate connecting strands, the plasmodesma, which traverse the walls. These represent in some cases the persistence of the original continuity of the protoplasm of two sister cells, but for the most part they are probably new contacts established when growth is completed. The presence of these delicate connections has been established in a large number of cases, and it is probable that they are of universal occurrence in the higher plants. They play a part in the correlation of the functions of the cells, and they may also provide the means by which stimuli can be conveyed through long tracts of tissue in those cases, where the region stimulated is distant from that in which the response is shown.

The distribution of plants can be considered from two main aspects. The present or past distribution of species, genera and families, termed floristic plant geography, can be studied both for its own sake and as a factor in determining the location of human activities or features of the earth's surface. But besides this purely geographical aspect the facts of distribution can be con sidered in relation to their causes; the influences of climate, of topography, of the soil conditions, of other plants, of animals, and even of the past history of the superficial crust of the earth and of the species itself. It is this relation of plants to their environment, in its widest sense, and the mutual effect of environ ment and organism, which is the province of ecological plant geography, or briefly plant ecology.

Floristic Distribution.

The floristic distribution of species exhibits the greatest possible variation from the species which as a wild plant is confined to a limited area (endemic) to wide-rang ing species such as many microscopic plants (e.g., some bacteria)

which are apparently universal in their occurrence. There are probably no vascular plants that are strictly cosmopolitan, but the common bracken (Pteridium aquilinum) and the reed (Phrag mites communis), though far less common in the tropical regions, are feral species in the most diverse climes (the former, however, notably absent from oceanic islands). In striking contrast are such plants as the fern Thyrsopteris elegans,—an endemic confined to the Island of Juan Fernandez or Pringlea antiscorbutica restricted to the tempestuous shores of Kerguelen.

Endemics are an especial feature of oceanic islands as is shown by the flora of Madagascar, which comprises some 4,00o species of vascular plants and contains no less than 75% of endemics. In New Zealand the proportion of endemics is 74%, whilst in Hawaii the endemics constitute 8o% of the entire flora. By contrast the floras of continental islands are practically destitute of endemics, as is shown by that of Britain, which amongst its flowering plants probably includes but one endemic of specific rank, namely, the wolf's-bane (Aconitum anglicum).

It is clear that when a new species originates it will not imme diately attain its climatic limits but its spread will be a gradual process, occupying a longer or a shorter time according to the efficiency of the means whereby the dispersal of its seeds or other reproductive bodies is attained. 'It is the belief that this process is in general secular rather than rapid, which has led to the view that the area occupied by a species, genus, or family is roughly depend ent upon its age. Willis has adduced evidence to show that the majority of endemics are species of recent origin which have either not had sufficient time to spread over a larger area (e.g., local endemics in continental areas) or have been prevented, by the presence of natural barriers, extending beyond their place of origin. The latter explains the high proportion of endemics on i oceanic islands. On the other hand, it is certain that some endemic species are relics of a former more extensive area of occupation, the remnants of a bygone vegetation. The fern genus Matonia is at the present day represented by three species, of which two are confined to Borneo and the third also occurs in the Malay Peninsula ; yet fossil evidence indicates that ferns of this affinity had an almost cosmopolitan distribution in Mesozoic times. The Gleicheniaceae, the Schizeaceae, and Marattiaceae, families of ferns now practically absent from the northern hemisphere, fur nish further examples of area diminishing with the lapse of time. Amongst the flowering plants the present restricted distribution of Ginkgo biloba, the tulip tree (Liriodendron tulipifera) and the sequoias, affords a marked contrast to the widespread occurrence of species of these genera in Tertiary times and shows the im portance of the past history of a group in appreciating the signifi cance of the existing representatives.