SCIENCE- G OS SIP. 



posed to the air, and as it presents a large surface 

 evaporation takes place, and some of the water is 

 withdrawn from the liquid in that part. In conse- 

 quence more water will be taken in from B, the 

 fluid in which will in turn become more concen- 

 trated ; and the process extends through c, D, and 

 E. The result of this is that the liquid at F, which 

 remains at the same concentration, will be con- 

 tinuously drawn up towards A, and an upward flow 

 of liquid is thus produced. The same process goes 

 on in the plant, the water evaporated from the 



escapes. At the same time, it must not be sup- 

 posed that the rate of transpiration remains con- 

 stant for any dne plant. It varies naturally with 

 the hygrometric conditions of the air, more water 



St 



Fig. 3. Separate elements of the xylem and phloem : — 1 1 1„ l a , 

 pitted and reticulate tracheides ; a.t., annular tracheide with 

 ring-like thickenings ; s.t., sieve-tube from the phloem : s. pi., 

 sieve-plate, the lower one seen in surface view, the upper in 

 section ; p., protoplasmic contents, etc., of the tube in a coagu- 

 lated and shrunk condition. 



4 

 leaves being replaced by some more of the dilute, 

 solution of earthy salts. It is this that constitutes 

 the transpiration current. 



We see, then, that the two chief factors in the 

 upward conduction of sap are root-pressure and 



Fit;. 4. Trans-section across a blade of a leaf in region of 

 midrib : — pp., epidermis ; p., palisade parenchyma ; sp., spongy 

 parenchyma ; b, bundle, with xylcm on upper phloem on under 

 aspect ; .?/., stomata. 



transpiration from the green parts of a plant. 

 Transpiration occurs in any part of a plant that 

 possesses stomata (see fig. 5) in its epidermis ; for 

 it is through these that the evaporated water 



Fig. 5. Section across 

 dermis ; g., guard-cells ; 

 parenchyma. 



stoma: — v., the vestibule; p., epi- 

 c, respiratory-cavity ; sp., spongy 



being evaporated into "dry " air than into " moist " 

 air. In this way the transpiration current varies 

 also in rate, and it will be found that plants will 

 only flourish in air that exactly suits their hygro- 

 metric conditions, although they may to some 

 extent become adapted to abnormal states of the 

 atmosphere. It is found also that even root- 



=J 



Fig. U. Diagram to illustrate the mechanics of transpiration : 

 for explanation see text, m, m are the walls of the tracheides 

 that are adjacent to one another. 



pressure varies periodical!}', the flow being more 

 active shortly after midday, and least in the early 

 hours of the morning. 



The elaborated sap in the cells of the mesophyll 

 of the leaf is conducted away from the leaf by 

 means of the phloem elements on the under side of 

 that organ, and, reaching the phloem of the stem, 

 part of it is directly distributed by osmotic pro- 

 cesses to the outlying cortex, another part being 

 conducted and stored by some peculiar elements 



M I 



