INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 753 



The separate crystals are not coloured by iodine-solution, but sucb a 

 solution assumes a rod colour, which turns to violet and finally blue 

 on evaporation. 



A similar difference in the degree of molecular cohesion occurs 

 also in the case of cellulose. On the one hand, the cellulose in certain 

 cotyledons is a delicate tissue, coloured blue by iodine and acted on 

 by diastase ; on the other hand, it is a hard solid mass, as in cotton, 

 the pith of the elder, &c., which undergoes change only after long- 

 continued action of concentrated acids or alkalies. 



Cellulose and starch present therefore many analogies, not only in 

 their chemical, but also in their physical properties. 



Rain of Sap.* — On the 22nd August, 1878, at 4 p.m., in calm 

 weather, with a clear sky, and a temperature of 24'^ C. in the shade, 

 M. C. Musset was struck by the evolutions of the gnats under the 

 outspread branches of two trees, varieties of Abies excelsa. Round 

 some yew trees {Taxus haccata), a lime tree {Tilia plati/pltylla), and 

 two very old stumps of Althcea frutex, he noticed similar swarms of 

 insects, but less numerous ; under other trees there was not a single 

 gnat. He then noticed an immense quantity of very limj)id little 

 drops falling in the form of fine rain, which became visible against the 

 rays of the sun. 



The phenomenon was observable for a fortnight, at any hour of 

 the day, and often, by the light of a lamp, in the night. 



If, on hot days, with a misty sky, the falling of the drops is not to 

 be seen, it is easy to prove the reality of the fact by spreading a piece 

 of dark-coloured silk material. 



The following are briefly, in his opinion, the approximate causes 

 of this phenomenon. At the end of summer and the commencement of 

 autumn, vegetation gradually suspends its activity, the tissues become 

 cuticularized, and consequently the transpiration decreases ; but the 

 sap continues to rise in the vascular bundles, and, being no longer 

 utilized for the work of assimilation, its excess is poured out by the 

 stomatic ojienings and the bordered pits, so peculiar to the cells and 

 vascular fibres of the Coniferae. 



This aqueous sap is nearly insipid, perhaps slightly purgative, 

 and colourless, but in a few days it assumes a j)ale amber tint. 



B, CRYPTOGAMIA. 



Cryptogamia Vascularia. 



ProthaUium of Salvinia natans.| — Bauke has investigated the 

 development of the unfertilized prothallium of Salvinia, a point left 

 undetermined in Pringsheim's investigations. Sachs has already 

 shown that of the two families of Ehizocarpese, the SalviniaccEe exhibit 

 an intermediate stage between the other family, Marsileaceee, and 

 ferns, especially in reference to the structure of the fructification. 

 The development of the unfertilized prothallium of Salvinia natans 

 was thus followed out by Bauke. The power of division of the cells 



* ' Comptps Reudu?,' Ixxxviii. (1879) p. 30G. 

 t ' Flora,' Ixii. (1879) p. 2U9. 



