I 



INFLUENCE OF TEMPERATURE ON VEGETATION. 733 



turgidity. This phenomenon is seen with remarkable clearness in the large leaf-stalks 

 of Cynara Scolymus when they freeze slowly. The succulent parenchyma separates 

 from the epidermis, which surrounds the former like a loose sack ; the parenchyma itself 

 splits apart in the interior so that each fibro-vascular bundle is enclosed in an envelope 

 of parenchyma. Fig. 473 shows how the coatings of ice project from the masses of 

 parenchyma. From pieces of the leaf-stalk which weighed 396 grammes I have collected 

 99 gr. of ice, which, when evaporated to dryness after thawing, left only slight traces 

 (about o-i p. c.) of solid substance. I have often observed similar phenomena in other 

 plants; the formation of ice is however not so regular as here. In the cavities of the 

 ruptured tissue (as in the succulent stems of the Cabbage) small irregular flakes of ice are 

 formed ; sometimes the ice splits the epidermis and projects in the form of combs above 

 the surface of succulent stems (Gaspary). I have already shown elsewhere ^ that when 

 sections of succulent parts of plants (such as the Beet) are protected from evaporation 

 and allowed to freeze slowly, continuous coatings of ice are produced on the surfaces of 

 the section, consisting of prisms growing at the base. The formation and growth of 

 these ice-crystals may be explained in this way. The temperature of the tissue falls to 

 a certain point, thereby causing the freezing of an extremely thin stratum of water 

 which overspreads the outside of the uninjured cell-walls. A new very thin stratum of 

 water then immediately passes out of the cell- wall to its surface and also freezes, 



Fig. 473— Transverse section of a slowly frozen leaf-stalk of Cynara Scolymus; e the detached epidermis ; g the 

 parenchyma in which lie the transverse sections of the iibro-vascular bundles (left white). It forms a tough but pliant 

 mass, which is ruptured during the process of freezing ; a peripheral layer has become separated from the inner parts 

 which surround the bundles ; the surface of each portion of the parenchyma is covered with a crust of ice ATA' con- 

 sisting of densely crowded prisms (the cavities of the ruptured tissue are left black in the figure). 



thickening the stratum of ice already formed; and thus it goes on. The cell-wall 

 is constantly absorbing cell-sap-water from within, and at the same time allows the 

 outermost molecular stratum of its water of imbibition to freeze. The first thin layers 

 of ice on the exterior of the uninjured cells form polygonal plates in contact with one 

 another; each plate becomes a prism by growth on its lower side; and the closely crowded 

 prisms form a coating of ice which easily crumbles. These processes cause tbe cell-sap 

 to become a more and more concentrated solution, while the cell- wall and the protoplasm 

 contain a gradually diminishing quantity of water. It can now be to a certain extent 

 understood why a rapid thawing kills the cells, while a slow thawing does not ; for if the 

 thawing take place slowly, the ice-crystals melt at their base where they touch the cell ; 

 the water as it becomes fluid is at once absorbed into the cell ; and the original con- 

 ditions of the cell-sap, cell-wall, and protoplasm may be re-established, if they have not 

 been permanently impaired during the freezing. If on the contrary the coating of ice 



* Sachs, Formation of Crystals in the Freezing, and change of the Cell-walls in the Thawing of 

 Succulent Parts of Plants (Bericht der kon. sachs. Ges. der Wiss, i860). I have already mentioned 

 in the first edition of this work the formation of crystals in the interior of frozen plants described 

 above, and applied it to the explanation of freezing. Prillieux (Ann. des Sci. Nat. vol. XII. p. 128) 

 afterwards, in 1869, also described similar phenomena in a variety of plants. 



