. Une introduction à la physiologie végétale. Physiologie végétale. 18 physiologie végétale trois dimensions, de sorte qu'une masse de protoplastes est formé, les progrès de la différenciation devient marqué. Dans une telle masse de la nécessité de fournir de l'eau pour toutes les unités constituantes implique des difficultés particulières qui varient selon l'environnement de l'usine en observation. Ceux qui vivent dans l'eau ont besoin de beaucoup moins d'arrangements complexes que ceux qui sont à la maison sur la terre, qu'ils peuvent absorber de l'eau de l'extérieur en général leur surface, et après l'absorption, il peut facilement se retrouver à partir de cellules

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. An introduction to vegetable physiology. Plant physiology. 18 VEGETABLE PHYSIOLOGY three dimensions, so that a mass of protoplasts is formed, the progress of differentiation becomes marked. In such a mass the necessity of supplying water to all the constituent units involves particular difficulties which vary according to the environment of the plant under observation. Those which live in water need much less complex arrangements than those which are at home on land, as they can absorb water from the exterior by their general surface, and after absorption it can easily make its way from cell to cell. Those which derive their supply of water entirely from the soil, as is the case with nearly all terrestrial plants, need a specialised mechanism for trans- port of the water after it has been taken up. On the other hand, the supply of a suitable atmosphere to the interior of the planT; for the service of its more deeply seated protoplasts is attended with more difficulty in the case of an aquatic than a terrestrial plant. In cell-masses, therefore, such as are found in all plants possessing more than microscopic dimensions, we meet with considerable differentiation of the plant-body. The explanation of the details of such differentiation is to be found in the division of labour which the size and the mode of life of the particular plant demand. The first indication of this differentiation in the vegeta- tive body of the plant is a change in the character of the exterior, which has for its object the protection of the plant from external injurious influences. This can be seen even among the seaweeds, simple as is generally the structure of mem- bers of this group. Fucus and its I allies, which form part of the class Fig. 19 —Piece of the l Thallub or Peivetia, of the brown Algae, have their external SHOWING CHABAOTEB OF -, , , the Extebnal and Sub- ceils much smaller, more closely put jacent cELLS. x 800. together, and generally much denser than the rest of their tiss

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