74 SUMMARY OF CUERENT RESEARCHES RELATING TO 



The author was not able satisfactorily to determine the function 

 of the sieve-tubes ; but inclines to the view that they serve for the 

 transport of insoluble substances from one part of the plant to another. 



Cork-growths on Leaves.* — E. Bachmann has studied the nature 

 of the cork-growths which appear on the leaves of many trees and 

 shrubs, including Hex, Zamia, Cryptomeria, Buscus, Eucalyptus, 

 Peperomia, Anthurium, and others, most commonly on the under side. 

 The layer in which the formation of cork commences is, in the 

 majority of cases, the first hypodermal layer, less often the epidermal, 

 less often still a lower layer ; from this layer it usually extends to 

 others also. The mature development of the tissue may be referred 

 to two types. In some cases the entire structure has the form of a 

 wart-like elevation projecting above the surface of the leaf ; all the 

 walls are then parallel to one another, and to the sui-face, and the 

 cells are arranged in very regular radial rows ; the whole tissue is 

 formed by the growth and divisions of a single layer. The second 

 variety of cork-structure occurs less often on the surface than in the 

 interior of the leaf. While originating from a single layer, the cork- 

 formation advances towards the interior ; the cork-walls are here also 

 tangential, but usually oblique to the surface of the leaf. AVhere the 

 parenchyma of the leaf contains large intercellular spaces, these become 

 filled with the cork-structure. In contrast to the cork-formation of 

 the stem, that of the leaf is by no means constant for the same genus 

 or even species ; even on the same leaf the structures of this nature 

 may vary in their origin and in their course of development. 



Heliotropism.t — In piu-suance of his previous investigations of 

 this subject, f Wiesner now publishes an elaborate paper, in which the 

 following are some of the more important results arrived at. 



In contrast to the ductility, the elasticity of the cell-wall decreases 

 from the lighted to the shaded side ; in organs with heliotropic cur- 

 vature, the tissue-tension occurs only between the epidermis and paren- 

 chyma ; later also in the latter. The heliotropic sensitiveness of an 

 organ is measured by the rapidity with which the turgidity of the cells 

 on the shaded side increases in contrast to that on the light side. It is 

 also greater the more ductile the cells remain on the shaded side, and 

 the less elastic the illuminated cells become. In opposition to the usual 

 theory, the author shows that in many cases, especially in plants that 

 are very sensitive to heliotropism, the most favourable conditions do 

 not occur in the maximum zone of longitudinal growth; strong 

 turgidity on all sides being a hindrance to heliotropic curvature. 

 The usual supposition that organs are especially heliotropic when 

 etiolated, is incorrect. Etiolated organs which are capable of growth 

 and susceptible to heliotropism, become still more susceptible when 

 slightly illuminated on all sides, depending on the decrease of 

 turgidity. Negative heliotropism must also be regarded as a pheno- 

 menon of growth. 



* Jahrb. wiss. Bot., xii. (18S0) pp. 191-235. 



t Deukschr. K. Akad. Wiss. Wien, xl. (1880). 



t See this Journal, ii. (1879) p. 593; iii. (1880) p. 984. 



