604 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of a piece of blotting-paper, the other end of which dips into water, to 

 come into contact with tan containing plasmodia of the Mthalium, he 

 found the latter to display the phenomenon of rheotropism,* i. e. they 

 move to meet the current of water, travelling in a horizontal or even 

 in a vertical upward direction. The same was observed with the 

 Plasmodia of a small species of Physarum. The plasmodia display 

 not only rheotropism, but also hydrotropism, i. e. movements regu- 

 lated by the distribution of water in the substratum, when this water 

 is not in motion. During the greater part of the period of develop- 

 ment they display positive hydrotropism, or are attracted towards the 

 source of water. They are indeed very dependent on water for their 

 development. On a uniformly moist substratum in an atmosphere 

 saturated with moisture, they spread uniformly in all directions ; 

 while in a dry air, when the substratum is gradually drying, they 

 contract, and collect in the dampest spots. Negative hydrotropism 

 does, however, also occur, where the sporangia bend away from the 

 damp spots and stand erect ; this was observed, but only rarely, in 

 sporangia of Physarum, Didymium, and ^thalium. 



Various chemical substances, such as crystals of sodium chloride, 

 nitre, cane-sugar, grape-sugar, drops of glycerine, &c., exercise a 

 repellent effect on the plasmodia. On the other hand an infusion of 

 tan produced an opposite attractive result ; and to this property of 

 moving towards the spots where the supply of nutriment is most 

 abundant Stahl gives the name trophotropism. The same substance 

 may have opposite influences of attraction or repulsion according to 

 the degree of concentration of the solution. 



As regards heliotropism, the author adds nothing to the facts 

 already known, that plasmodia move from illuminated spots to those 

 that are in shade. He was unable to determine satisfactorily their 

 relation to geotropism ; the vertical position of the fructifications of 

 Myxomycetes appears to be due rather to hydrotropism than to 

 geotropism. 



With respect to the effect of heat, if JEthalium is exposed on the 

 two sides to unequal temperatures, an evident motion takes place 

 towards the warmer side. 



Lichenes. 



Cephalodia of Lichens.t — Pursuing this subject, K. B. J. Forssell 

 states that the algse found in connection with cephalodia belong to 

 all the groups of Phycochromacese, including the following families : 

 — Nostocacese, Sirosiphonea3, Scytonemacese, Chroococcaceae, and 

 Oscillariacesa, the first being the most largely represented. The 

 Nostoc cells take up very different positions in relation to the gonidial 

 layer of the thallus, belonging to the different kinds of cephalodia 

 already described. Occasionally several species of algse are found 

 in the same cephalodium. 



The development of cephalodia is always the result of the mutual 



* See this Journal, ante, p. 413. 



t Flora, Ixvii. (1884) pp. 33-46, 58-63, 177-87. Cf. this Journal, ante, p. 100. 



