CELLULAR TISSUE. 337 



Variously-cut sections of leaves should be made, and slices 

 taken parallel to the surfaces at different distances, for the 

 purpose of microscopic examination. 



Among the cell-contents of some plants, are beautiful 

 crystals called Raphides : the term is derived from pacf>t<g, 

 a needle, from the resemblance of the crystal to a needle. 

 They are composed of the phosphate and oxalate of lime ; 

 there is a difference of opinion as to their use in the 

 economy of the plant. 



Mr. Gulliver has insisted upon the value of Eaphides 

 as characteristic points in many families of plants. 



He observes that doubts as to the value of raphides as 

 natural characters and as to their importance in the vege- 

 table economy at all will be entertained by those who do 

 not clearly distinguish between raphides and sphssraphides. 

 Schleiden asserts that the "needle-formed crystals, in 

 bundles of from twenty to thirty in a cell, are present in 

 almost all plants," and "that inorganic crystals are rarely 

 met with in cells in a full state of vitality." 



He further states that so really practical is the presence 

 or absence of raphides, that by noticing them he has been 

 able to pick out pots of seedling Onagracese, which had 

 been accidentally mixed with pots of other seedlings of 

 the same age, and at that period of growth when no 

 botanical character before in use would have been so 

 readily sufficient for the diagnosis. 



If we examine a portion of the layers of an onion, fig. 

 183, No. 1, or a thin section of the stem or root of the 

 garden rhubarb, fig. 183, No. 4, we shall find many cells 

 in which, either bundles of needle-shaped crystals, or 

 masses of a stellate form occur, not strictly raphides. 



Raphides were first noticed by Malpighi in Opuntia. 

 and subsequently described by Jurine and Raspail. 

 According to the latter observer, the needle-shape or 

 acicular are composed of phosphate, and the stellate of 

 oxalate of lime. There are others having lime as a basis, 

 in combination with tartaric, malic, or citric acid. These 

 are easily destroyed by acetic acid, and are also very soluble 

 in many of the fluids employed in the conservation of ob- 

 jects; some of them are as large as the l-40th of an inch, 

 others are as small as the l-1000th. They occur in all 



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