502 DEVELOPMENT OF LIGHT AND HEAT. 



of the flowers also rises, as shown by the following observations which were con- 

 ducted in a place in the garden shaded from the direct influence of the sun's rays: 



8-8 ... 15-2 ... 17'7 ... 20-0 ... 21'2 



of the Eed Foxglove, 



Corresponding temperature of the surrounding air, 8'7 ... 15'0 ... 17'2 ... 19'1 ... 19'5 

 Difference, 0'1 ... 0'2 ... 0'5 ... 0'9 ... 17 



While the liberation of heat occurs in all living plants, and is a natural con- 

 sequence of respiration, i.e. of the combustion of carbon compounds, the development 

 of light, which in other respects appears to be in many ways connected with the 

 processes of combustion, is observed in living plants but seldom. It is only recog- 

 nized with certainty in the Hymenomycetes, a group of fungi in which the rise of 

 temperature during respiration has already been described. But even of these 

 Hymenomycetes only relatively few are luminous, and these few only in certain 

 stages of development. Most frequently the luminosity occurs in the mycelium of 

 mushroom-like forms (Agaricinece), which permeate the wood of old tree- trunks 

 and the creeping roots of trees on the surface of the damp forest ground. This 

 mycelium forms thicker dark strands, frequently joined together by cross-connections, 

 which penetrate principally between the wood and the cortex, and these form most 

 characteristic nets and lattice- works; it also consists of very slender dark threads, 

 which take up their position in the wood usually at right angles to the long axis of 

 the trunk; and, finally, there are extremely delicate colourless threads which grow 

 through the woody cells in the manner shown in fig. 32. These actually per- 

 meate the entire wood, and are only perceptible to the naked eye when they are 

 woven into net- works, and then are seen as whitish fringes and membranes situated 

 on the sides of the holes formed in the disorganized wood. 



It is these fine threads and webs of the mycelium which exhibit the remark- 

 able illumination. Where they completely invest the wood-cells, it looks as if 

 the wood itself were luminous, and we commonly speak of luminous wood and 

 the luminous decay of tree-trunks. There is no doubt that the luminosity is ex- 

 hibited by the mycelia of various agarics, which destroy the wood of firs and 

 other foliage-trees. Usually the Rhizomorph (Agaricus melleus) is alone pointed 

 out as the cause of the luminosity in wood, since this species is widely distributed; 

 and where it has established itself sends up every year many receptacles, so that 

 there is no difficulty in determining the species. But since luminous wood is also 

 observed in the pine forests of higher mountain districts where the Rhizomorph 

 is no longer found, it must be concluded that the mycelia of various other agarics, 

 whose species cannot be determined on account of the absence of fructifications, 

 exhibit the same phenomenon. The light is best seen in the open, in midsummer 

 and autumn, after many days of wet weather, when the wood permeated by the 

 mycelium has been moistened by the rain. But the moisture absorbed by the 

 wood must not exceed a certain amount. Too much saturation prevents the 

 phenomenon of luminosity just as much as excessive dryness. If the wood is 

 removed from the place where it shines so well, the luminosity rapidly diminishes, 



