33° 



SCIENCE- GOSSIP. 



containing portions, " xyleni " or wood, and phloem, 

 with a layer of meristematic tissue, the cambium, 

 between. Of these the xyleru forms the tissue which 

 conducts the raw sap upwards towards the leaves ; 

 the phloem forming a tissue whose function is to 

 distribute the elaborated sap from the leaves to 

 the cortex and other parts of the plant. 



In the ultimate ramifications of the roots, the 

 raw sap passes from the root-hairs to the central 

 cylinder of the root by a process of osmosis, and 

 here gets through into the elements of the xylem. 

 These are in young roots of a simplified nature, and 

 consist for the most part of elongated elements 

 with spiral or annular thickenings upon the walls 

 (see figs. 2 and 3). Further up we get, on account 

 of secondary thickening, additional xylem elements 

 of many kinds, such as the forms with pits, reti- 

 culated walls, and others. All these fall into the 

 category of " tracheides,'' and are designated as 

 annular, spiral, or reticulated, as the case may be 

 (see fig. 3.) 



We must consider, How is the raw sap able to 

 pass up this long line of xylem elements ? The 

 answer is that the xylem, with its system of 

 tracheides, many of which possess numerous pits 

 in their walls, affords a transmitting channel for 

 the sap. It was formerly thought that the walls 

 of the tracheides were traversed by the fluid, but 

 this has been proved not to be the case. It passes 

 by means of the spaces, and by them only ; more- 

 over, it has been shown ( 2 ) that the air in these 

 spaces exists there under a negative pressure, so 

 that by means of certain other forces that will be 

 mentioned later (root-pressure and transpiration) 

 the sap easily penetrates from one space to another, 

 and so upwards. 



If we take a short piece of hazel stem, cut both 

 ends flat, and block out the pith at either end by 

 means of some impermeable varnish, we can 

 demonstrate the extreme facility with which water 

 passes along the wood. Immerse one end in some 

 water, and observe the other end closely. In less 

 than one minute, if the piece is not too long 

 (10 cm.), the cut surface will become wet and 

 small drops of water will appear. If the piece is 

 taken out and held upside down, the water re- 

 maining on the uppermost surface will sink down 

 and appear on the lower surface, being, in fact, 

 aided in this case by gravity. We thus see that 

 the wood is highly permeable, and the fact that 

 the spaces in the xylem elements contain bubbles of 

 rarefied air, surrounded by a thin aqueous film, 

 would of itself be sufficient to explain the rapid 

 upward suction or attraction by means of the sur- 

 face tension force called into play. Moreover, the 

 existence of a large number of pits between 

 adjacent elements forms a great factor in the 

 upward conduction. These pits, it must be re- 

 membered, are not actual apertures, but are closed 

 by the thin "middle lamella "that is present between 

 (2) Detrner and Moore, " Practical Plant Physiology." 



adjacent cell-walls. This thin lamella is highly 

 permeable to a watery solution of salts. 



With regard to the physical forces brought into 

 play in aiding the rapidity and continual upward 

 flow of raw sap, two, namely "root-pressure" and 

 "transpiration," must be especially mentioned. The 

 first of these is dependent upon that pressure that 

 is set up during osmosis, and known as " osmotic 

 pressure," which in itself is dependent upon the 

 partial semi-permeability of the cell-wall and ecto- 

 plasm. It has, moreover, been supposed that at 

 certain points in the upward progress certain cells 

 exist whose upper wall is more permeable than 

 the lower wall to the same solution, and that these 

 form fresh relays, whereby new force of pressure is 

 attained. ( 3 ) 



The best way of demonstrating root-pressure is 

 to procure a rapidly growing plant in a pot and cut 

 off its upper foliage. To the end of the cut stem 

 affix by means of a short rubber tubing a mano- 

 meter, the space between cut surface and mercury 

 being filled with oil. In a few hours the level of 

 the mercury in the two limbs will be found to be 

 different, indicating an increase of pressure on 

 that side connected with the cut stem. The ex- 

 periment is easy to perform, and shows us what 

 a considerable force may be obtained from this 

 pressure, a force that is constantly employed in the 

 forcing-up of sap. 



Although root-pressure forms a powerful factor in 

 sap transmission, " transpiration " is even more effi- 

 cacious. To study this we must first of all examine 

 the terminations of the fibro-vascular bundles in the 

 leaves, which are the main organs of transpiration. 

 A section across the lamina of a leaf near its 

 junction with the petiole will cut across several of 

 the leaf-bundles as they come up from the stem. 

 It will be seen on microscopical examination they 

 are of a simpler formation than those in the stem 

 or root, the xylem consisting of merely a few 

 scattered annular or spiral tracheides, situated 

 towards the upper surface of the leaf, and a small 

 mass of succulent phloem on the under surface 

 (see fig. 4). A thin layer of cambium may be 

 present if near the petiole. The ultimate termina- 

 tions in the mesophyll of these bundles consist of a 

 few spiral tracheides, surrounded by a layer of 

 elements that resemble embryonic phloem-cells ; 

 and these tracheides stretch in amongst the 

 succulent, turgid cells of the middle layer, or 

 " spongy parenchyma " of the leaf. In order to 

 fully understand the process that goes on during 

 transpiration we may inspect fig. 6, in which A 

 represents a cell of the spongy parenchyma ; B, c, 

 D. and E are single elements of a fibro-vascular 

 bundle, B being the terminal tracheide ; and F is 

 the soil, with its dilute solution of salts. 



Let us now start with an equally concentrated 

 solution of salts in all these receptacles. A is ex- 



(3) The fact that in the ultimate rootlets the cells become 

 turgid also constitutes a great aid to root-pressure. 



