MICROSCOPIC OBJECTS. 75 



glands in recent woods are excellent tests of the defining 

 power of a microscope : they are about -5^0^^ ^^ ^^ 

 inch in diameter. A magnifying power of 100 diameters 

 exhibits them very weU. 



Raphides. — If the leaf or bulb of a common Hyacinth 

 be wounded, a discharge of fluid ensues ; if this fluid be 

 received upon a glass shde and submitted to the micro- 

 scope, a number of minute acicular bodies will be observed 

 floating in the hquid ; these are called Raphides. They 

 are common in many Monocotyledonous plants : they 

 depolarize Hght, and are mostly composed of oxalate of 

 hme. 



Cuticles. — The external covering of plants, called the 

 Cuticle, consists of a thin membrane adherent to the cel- 

 lular tissue beneath it. Under the microscope it appears 

 traversed by lines in various directions, giving its surface 

 a reticulated appearance. The form of these reticulations 

 varies very much in different plants : in many they are 

 hexagonal; in others they are prismatic ; while in others, 

 again, no determinate geometrical figure can be assigned 

 to them. 



If the cuticle of the under side of a leaf of Candytuft 

 (Ileris umhellata) be carefully examined under the micro- 

 scope, among the sinuous reticulations will be observed, 

 darker oval spots ; these are called Stomata, and are the 

 orifices by which a function analogous to respiration 

 in animals is efiected. They also serve for the exit of 

 water from the plant by means of evaporation. Plants 

 destitute of stomata, as the South American Cacti, will 



