32 



METABOLISM 



that, according to differences in temperature, gelatine also shows variations 

 which are very noticeable, but always less so than those exhibited by protoplasm 

 in its resistance to the passage of water. Nevertheless it appears to us that the 

 part played by the protoplasm in the absorption of water is not purely physical. 

 It is certainly remarkable that the rate of water transference increases only 

 slowly between 20° and 30°, and it is unfortunate that Rysselberghe did 

 not employ still higher temperatures in his researches, seeing that, as we shall 

 have frequent occasion to note in the course of these lectures, the vital activity 

 of the plant increases in intensity proportionally as the temperature increases, 

 until a degree is reached somewhere between 30° and 45°, when it again de- 

 creases. Certain observations of Kosaroff pertinent to this question tend to 

 support this view. Withering soon takes place if a stream of carbon-dioxide 

 or hydrogen be passed through soil in which healthy pot plants are growing ; 

 absorption of water is therefore retarded by this means as well as by low tem- 

 peratures. This result may be observed within an hour after the initiation 

 of the experiment, and the carbon-dioxide itself could scarcely have caused 

 death in that time. The hydrogen is not so rapid in its action ; we know for 

 certain that it is itself harmless, but operates only by displacing the oxygen. 

 It would appear, therefore, from the evidence afforded by these researches, 

 that the absorption of water is retarded by suppressing the supply of oxygen 

 to the root. Oxygen, as we shall see later, is an indispensable factor in a large 

 number of vital processes, although of no account in the diffusion of water 

 through a dead membrane ; we are thus compelled to believe that living 

 protoplasm plays a great, but as yet unknown, part in the absorption of water. 

 The fact established by Kosaroff that dead roots take up less water does 

 not necessarily prove this, for death itself will doubtless induce purely physical 

 changes in the plasmatic layer. 



Biologically it is of the greatest interest to note that all plants are not 

 injuriously affected to the same degree by low temperatures ; many, indeed, 

 can still absorb water from a frozen soil (Kosaroff, 1897). 



The root is the normal organ for absorption of water in ordinary land 

 plants, hence plants whose roots have been destroyed invariably die from 

 want of water, even though their shoots be frequently watered by rain or 

 dew. We must not, however, conclude from that, that aerial organs are al- 

 together unable to absorb water. The epidermal cells of the leaf, just as much 

 as those of the root, contain osmotically active substances in their vacuoles, 

 and must also be able to absorb water if only the outer wall be permeable 

 to it, and if sufficiently large quantities of water derived from rain or dew can 

 accumulate on the leaf. Not infrequently, however, the shape and arrange- 

 ment of leaves (Stahl, 1893) are adapted to the rapid drainage of surplus 

 water from the leaf surface. Thus, according to Stahl's researches (1897), the 

 position of leaves on the axis is such as to prevent or hinder the formation of 

 dew upon them (Lect. XXXIX) ; finally anatomical adaptations occur, especially 

 coatings of wax, which render the leaves waterproof. Such peculiarities in 

 structure are of restricted occurrence, but aerial parts of plants differ in general 

 from subterranean parts in possessing a cuticle formed from the outermost 

 layer of the cell-wall. This cuticle is not only of maximum thickness on the 

 stems and leaves, but exhibits varied physical and, probably also, chemical 

 peculiarities. The cuticle of the shoot only has hitherto been studied in suffi- 

 cient detail, that of the root is still urgently in need of investigation. A 

 research of this kind has been carried out by Kromer (1903, Bibl. botan., 

 Heft, 59) in the Marburg Institute, and he holds that the root epidermis has an 

 outer wall of very varied composition and structure, but that a genuine cuticle, 

 such as occurs in the epidermis of the stem and leaf, is entirely absent. 



The cuticle has been found to consist of a material which shows a strong 

 resemblance to cork, agreeing with that substance in possessing the physical 



