INFLUENCE OF TEMPERATURE ON VEGETATION. 6rr 



and succulent stems frozen at a temperature between - 5^^ and - 10^ G. it is easily seen 

 that only a portion of the water is present in the form of crystals of ice ; another portion 

 permeates the cell-walls which are not rigid but still flexible. If the congelation takes 

 place slowly, the water assumes on the surface of the succulent tissue the form of a 

 coating of ice consisting of densely crowded small crystals. These crystals stand at 

 right angles to the surface of the tissue, and increase by growth at their base. A 

 very large portion of the water of a tissue may in this way take the form of a coat- 

 ing of ice, while the tissue, becoming less watery, contracts in proportion >, and loses its 

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

 of Cyjjara 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. 444 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 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 



Fig. 444.— Transverse section of a slowly frozen leaf-stalk of Cyna}-a Scolymus: e the detached epidermis ; i^ tlie 

 parenchyma in which lie the transverse sections of the fibro-vascular bundles (left white). It forms a tousjh 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 parencliyma is covered with a crust of ice A" A' con- 

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



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 (Caspary). 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 from the thickness of the cell-wall to its surface and also 

 freezes, 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 



^ When this contraction operates unequally on different sides of a leaf or branch, it is easy to 

 see that curvatures must result which are indeed actually frequently observed. The splitting of the 

 trunks of trees in consequence of frost is probably only the result of changes of this nature. 



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

 Succulent Parts of Plants (Bericht der kon. siichs. 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 1S69, also described similar phenomena in a variety of plants. 



