SCIENCE-GOSSW. 



MECHANICS OF CONDUCTION OF SAP. 



By Harold A. Haig. 



A S we trace upwards through the vegetable 

 -*-*- kingdom the various modes of life of different 

 plants, from the unicellular organism low down in 

 the scale to those of higher grades of evolution, 

 we find a gradual differentiation of the methods 

 whereby the fundamental life-factors — viz. absorp- 

 tion, respiration, nutrition, and excretion — are 

 performed ; and also of the agents that are, 

 during these processes, called into requisition. 



The first of these, absorption — including con- 

 duction — is one in which we can find, perhaps, 

 the most complete variation as regards the acces- 

 sories that are brought into play ; but here again, 

 as in most of the others, we can recognise the 

 same underlying principles. What strikes one 

 most is, in fact, the obvious division of labour 

 that obtains in the higher plants, a division which 

 requires that many tissues shall take part in the 

 same common work, whereas in the lower organ- 

 isms the whole of it may be thrown on to one 

 tissue, or even on to one special portion of a single 

 cell. 



I propose here to study the mechanism of 

 absorption and conduction of that most important 

 element in the higher land-plants — namely, the 

 sap — the word " sap " meaning that portion of the 

 more fluid constituents of a plant whereby all the 

 inorganic and most of the organic food-materials 

 are conducted and distributed. 



We may, for convenience, class this fluid into 

 two distinct portions — the first of these being the 

 " raw sap," that part which is taken in and con- 

 ducted upwards by the root and stem ; the other 

 part being that which forms the " elaborated " sap 

 that is derived from the leaves and other organs 

 of the plant in which assimilation is going on. It 

 is well to bear in mind that this distinction is in 

 no wise meant to trespass on the chemical nature 

 of these two, the intention being only to separate 

 them for greater simplicity of description. 



We now proceed to the consideration of the 

 manner in which the raw sap enters a plant ; to do 

 this we must first of all examine with the micro- 

 scope the delicate roots of some seedling ('). Under 

 a low power we can easily make out, a short way 

 from the tip of one of these roots, a portion that 

 is covered by a great number of hair-like processes. 

 Under a higher power (600-800 diams.) these pro- 

 cesses are seen to have a definite structure, and to 

 be prolongations of the cells of that outermost 

 layer of the root, the "epiblema," which corre- 

 sponds to the epidermis of the stem. It is not 



(1) These roots must be well washed, in order to detach 

 particles of adherent soil. When this has been done they may be 

 examined microscopically in the usual way. 



every epiblema! cell in this area that sends out 

 processors, or "'root-hairs," like this, as may be 

 evinced from the fact that many of them are quite 

 like ordinary epidermal cells, though perhaps some- 

 what more elongated. Each root-hair may be thus 

 seen to consist of a delicate cell-wall and a very 

 delicate layer of parietal protoplasm that sends 

 " bridles " across from one side to the other, and a 

 well-marked nucleus somewhere in the hair, or 

 may be in the originating cell. With a careful 

 preparation the characteristic "rotation" of the 

 protoplasm may be seen, but it is slow. This 

 structure thus identified, we at once proceed to the 

 most interesting and essential factors in the phe- 

 nomenon of absorption. 



We know from physical principles that a fluid 

 of a certain degree of concentration enclosed in a 

 tube, one end of which is closed by a membrane 

 permeable to that fluid, will, if placed under these 

 conditions in a beaker of the same fluid, but of 

 less concentration, take in a certain volume of 

 liquid until an equilibrium is set up. Further, if 

 the open end of our tube is connected with a 

 manometer, we can measure the pressure that 

 arises from this infiltration. It is, however, prob- 

 able that at the same time some of our fluid in 

 the tube will have escaped through the membrane 

 in an opposite direction. Nevertheless, since 

 in this case infiltration is so much more rapid than 

 filtration in the opposite direction, we get a certain 

 pressme, and this in the cell constitutes the so- 

 called " turgidity." In fact, there is a powerful 

 force at work which, as it were, draws in the fluid of 

 lesser concentration ; and it depends entirely on the 

 difference in concentration whether fluid shall 

 proceed in the one or the other direction. The 

 process in hand is known as " osmosis," and it is 

 this that determines the inflow into the root-hairs 

 of the dilute watery solution of earthy salts, 

 otherwise the raw sap. As a matter of fact, some- 

 thing does escape into the soil from the sap inside 

 the walls of the root-hair, and so we have not in 

 this case what physicists know as a " semi-permeable 

 membrane" — namely, a membrane that allow> a 

 certain substance in solution to pass in one direc- 

 tion only. It is, however, a very near approach to 

 this condition, and, as we shall see, there is some- 

 thing — namely, the "power of selection" — that acts 

 in much the same manner. This selective power 

 depends upon the layer of parietal protoplasm, or. 

 as it has been often called, the " primordial utricle*' 

 (Yon Mohl), some substances in solution being 

 allowed to pass through the cell-wall, but not 

 through the protoplasmic lining : some others 

 through this latter as well. In this manner a 



