i? SmitH: THE HISTOLOGY OF CERTAIN ORCHIDS 
the material secreted is deposited in a central vacuole rather 
than being poured out into ducts as in the secreting glands of 
animals or being deposited between the plasma membrane and 
cell wall as is the cacti. During the later development of the 
slime cell, the assimilation of material other than water may be 
also going on if material (starch or sugars) from the adjoining 
cells is being made into mucilage. 
The formation of raphides may be primarily dependent upon 
the supply of calcium in the soil (Kohl) and upon the supply 
of nitrates in the soil (Czapek). Oxalic acid may be formed in 
vitro by the oxidation of many carbon compounds with nitric 
acid, Czapek points out a connection between nitrates and 
oxalic acid in plants, saying that in certain forms it has been 
demonstrated that oxalic acid is produced abundantly when 
nitrates are given in excess, but that when nitrates are withheld, 
otherwise flourishing plants contain at most only slight quan- 
tities of oxalic acid. 
s to the function of raphides, various views are held. Stahl 
believes them to be a protective device to prevent plants being 
eaten by animals, but subsequent studies by Leuvin (1901) have 
proved this theory unfounded. Groom (1896) suggests that the 
precipitation of calcium oxalate neutralizes the poisonous effect 
which potassium oxalate has upon plants. Czapek considers 
that in some cases at least raphides serve as reserve food stuff. 
In support of this view he cites Sorauer’s report that these 
crystals disappear in the living cells of Orchis tubers, as well as 
DeVries’s observation on ripening potato tubers and his own 
observation of the same phenomenon in Begonia cuttings. 
The classification of plant slimes rests so far upon super- 
ficial appearance rather than upon inherent characters. This 
kind of grouping is probably responsible for the confusion 
in the literature dealing with them. Yet since the time of 
Frank (1867) three types have been clearlyrecognized. Tschirch 
(1889) substitutes “Schleime-epidermis” for Frank’s inter- 
cellular class; Walliczek (1893) accepts this classification, finding 
great difficulty, however, in being definite about any slimes 
which have certainly been proved to belong to the cell content 
slime, including in this class only that which occurs in Sym- 
phytum Orchis tubers; Kraemer (1908) recognizes cell 
content mucilage arising as a product of the protoplasm, and 
cell membrane mucilage derived from cell walls. 
