ways in interest, those from other regions as to warrant our speak-
ing of this Chinese invasion as the dominant feature of plant
introduction since the opening of the 2oth century.
Explorers have discovered the richest relic of the Palaearctic flora, its richness conditioned by an unique assemblage of deeply riven, snow-clad mountain ranges traversed by three mighty rivers Salween, Mekong, Yangtse of parallel course north to south separated by narrow divides, also deeply incised, across the monsoon trend. A paradise of species in the making! The first exploration with declared horticultural aims, tempted by records of the finds of French missionaries and Henry's wonderful Ichang collections, was that of Wilson, in 1 899, who made Western Hupeh and Szechwan his field of work during the succeeding 14 years, later passing east- wards to Korea and Formosa. He was followed in 1904 by Forrest still exploring who took Yunnan and adjacent Tibet and N.-E. Burma for his sphere. Through these pioneers thousands of new species have come to our ken. Later explorers who have affected horticulture have been Purdom in Kansu, Kingdon Ward still exploring in the same area as Forrest, Farrer in Kansu and, later with Cox, in N.-E. Upper Burma. Among the horticultural prizes and the plants of economic value that have come to us through these explorations in the shape of shrubs and trees, there are some two to three hundred new species of rhododendron alone, and amongst herbaceous plants primula gives us far over a hundred novelties. Genera to name a few that are prominent in the number of new species of shrubs and trees, indicating clearly the nature of the flora and the plants added to cultivation, are : acer, alnus, berberis, betula, buddleia, carpinus, clematis, cornus, corylus, cotoneaster, crataegus, deutzia, euonymus, fraxinus, hydrangea, ilex, ligustrum, litsea, lonicera, magnolia, photinia, pieris, populus, prunus, pyrus, quercus, rhododendron, rosa, rubus, salix, smilax, spiraea, styrax, syringa, tilia, vaccinium, viburnum, vitis; of conifers, abies, keteleeria, picea, pinus, tsuga. Similarly amongst herbaceous plants some of the noteworthy genera are: aconitum, adenophora, allium, androsace, anemone, aster, codonopsis, corydalis, cremanthodium, cyananthum, delphinium, didissandra, dracocephalum, gentiana, impatiens, iris, liliiim, lysimachia, meconopsis, nomocharis, oreocharis, pedicularis, pleione, polygonum, potentilla, primula, rheum, roscoea, saxifraga, sedum, senecio, silene, trollius, thalictrum.
To the west of this partially explored region lies a vast area un- explored extending to Bhutan, whence, ere long, riches, perhaps in diminishing amount, will be gathered. Cooper at the Himalayan end of this area has done splendid exploration work over Griffith's ground in Bhutan and further east, enriching horticulture with many good plants.
Apart from the new species which these explorations have brought to horticulture, two biological problems of horticultural interest are touched especially by the work of Forrest. One is raised by the modification of form observed in the direction of specific differentia- tion exhibited by single phyla in relation to the multiplicity of environmental conditions offered by the exceptional physical con- struction of the region. It suggests possible results positive or negative ^bearing upon evolution through correlation of plants in nature with similar forms in cultivation. The other that of humus plants growing on limestone immediately concerns horti- culture in view of the fact that rhododendron, for instance, cannot be grown in cultivation upon a limestone soil. That so many of the rhododendrons collected by Forrest have their leaves densely covered by a penetrating mycelium makes the suggestion admissible that the fungus of mycorrhiza has migrated from the uncongenial lime-soil environment to the leaf to function there forming a mycophyllon as a nitrogen-adjuvant. The following-up of these discoveries is for the future. (I. B. B.)
VIII. General Morphology. The wider problems of the origin of plant life on this world, its relation to animal organism, and above all the evolutionary progression of the flora of the land surface, have claimed the attention of successive generations of botanists. The older deductions of the Hofmeister school were admirably and lucidly summed up and amplified by Bower (1908) in The Origin of a Land-Flora, and this book has so held its own as a text-book that there has been little to add to it. In a posthumous volume Arber (1920, Devonian Floras) attempted even to visualize the actual geological epoch at which the transition took place from archaic aquatic algae to the first types of land-vegetation, as expressed in the change from a Lower Devonian flora of Thallophytes to the Upper Devonian of Archaeopteridae. Apart from the intensive investigation of the vestigial races of Pteridophyta of the present day, or of the recent debris of Palaeozoic times, it is possible to approach the subject indirectly, and to state the nature of the problems to be solved from the new view-points opened up in connexion with the earlier phases of plant life on the world surface by the consideration of conditions of life in the sea.
Since early papers by Luther (1899) and Bohlin (1901) on the relation of the reproductive cells of fresh-water algae to flagellates, the Flagellar theory has entered on a new phase to the extent that it is now freely accepted that all phyla of plant-life, as also all animals, must be based on a flagellate ancestry: that motile reproductive cells have not been evolved specially for the reproductive purposes which they serve, but indicate the retention of an older phase of aquatic existence. To this may be added the recognition by pro- tistologists (Doflein, 1916) of the fact that the autotrophic (plant) flagellate must be regarded as the precursor of all heterotrophic and animal flagellate phyla: while the vast variety of marine or- ganism in which the flagellated phase is still dominant or readily regained in reproductive stages implies that it is to the pelagic flagellates that one must look for conceptions of the origin of higher organism, rather than to the Amoeba of fresh water or sea bottom. As shown by Pascher (1917), the amoeboid habit may be attained secondarily, in connexion with available substratum, in any line of elementary flagellate evolution. In this way the conclusion appears inevitable, not only that life as we know it arose in the sea, but from the material of sea water, as the physico-chemical constitution of protoplasm suggests at the present day. Since no other factors but those of pelagic water and solar radiation are required to deter- mine the physiological and structural response of such living zoi'ds, a phase of continuous deep water over the entire world surface must have obtained to give rise to such " plankton " organism. The cell- unit, of which all higher life is composed, thus represents the soma attained in such an environment, established for all time with nuclear mechanism and faculty for division and fusion, as also all plasmic functions and assets inherited as the cell equipment of plant and animal organization.
It is to the sea that one must look for the incipient syntheses of early life, and the introduction of land or sea bottom within the photic zone will lead to the progression of attached organism (hor- mon) which responds to the more elaborate factors of water plus substratum. With the assumption of a sessile habit on the part of originally free zoi'ds of the plankton, the attached plant or animal proceeds to a benthic phase of existence, and in the case of auto- trophic zoi'ds it begins to be possible to define the scope of algology. The cell soma becomes enlarged and multiple as it successfully solves the problem of increased nutrition by a surface area exposed to a medium which is constantly renewed so long as the capacity for attachment is unimpaired. As opposed to the successful detachment of predatory animal organism (nekton), the detached autotrophic plant fails from impoverished nutrition. The development of the algal soma thus follows the infinite series of compromises between maximum surface for nutrition and minimum exposure to mechan- ical strain, from quiet dark levels to illumined surface-zones of rough water, giving rise to morphological differentiation of branch- ing axes, growing points, laminar extension, and ultimately to mas- sive highly differentiated shoot-systems with ramuli subserving attachment which come within the more popular connotation of plant-form. In all such cases, however, exigencies of racial continua- tion imply_ a resumption at some period of the older plankton- soma, and in this phase phenomena of sexual fusion may be main- tained; to attain a more complex differentiation (sex-distinction) as the wastage of protoplasts in regaining the sessile condition, on a substratum increasingly occupied in a violently agitated medium, may be brought under control. The development of algal form and volume commonly runs parallel with increasing specialization of sexual and asexual reproductive mechanism. The latter implies that a cytological alternation may be requisite in the life cycle. Highly specialized growth-forms of the benthic soma of autotrophic plants parallel the equally advancing benthic somata of holozoic nutrition (sponges, hydroids); and in both very comparable differ- entiations of sexual cells, sexual organs, and the retention, at any rate on the part of the male gamete, of the older flagellated soma illustrate the parallelism of the biological problems.
From such highly organized somata of the sea the flora of the land takes origin, both as compulsory transmigrants on the first exposed land-surfaces, and as left residual in water now renewed by atmospheric precipitations as " fresh," and devoid of much of the essential food salts. Starvation in fresh water and desiccation on dry land become the determining factors of all advancing land-vegeta- tion i ; though in the case of the latter the implied light-supply may be far in excess of older photic relations, as the oxygen capacity of the ' atmosphere again is beyond the available free supply of the water. The insistent problems of the land plant are mechanical support and orientation in the lighter medium of the air, protection from ex- treme loss of water, absorption and conduction from an attachment- surface following separation of the absorptive and photosynthetic tracts, and the adjustment of older reproductive organs to the exigencies of dispersal by air-currents instead of by moving water. In this way the inherited equipment of the algal soma is specialized and amplified to meet the new requirements. An epoch in which such natural selection may be rigorous over long-continued ages of slow progression and regression, has been visualized as a period of " Transmigration," effected in situ, as the sea-bottom may be partially exposed or again covered by oscillating changes in the earth's crust over geological epochs, as the net result of foldings of the surface-layers. Older laminar ramuli of algae attain further
