D2 A TEXTBOOK OF GENERAL BOTANY 
by comparing Fig. 17 with Fig. 20, which represents a section 
through the spongy parenchyma parallel with the epidermis. 


Fig. 22. Raphides from leaves 
Left, taro (Colocasia esculentum) ; right, Pistia stratiotes. (x 160) 
Owing to the fact that the chloroplastids are closer together in 
the palisade chlorenchyma than in the spongy chlorenchyma, the 
upper surfaces of many leaves appear to 
be deeper green than the lower surfaces. 
Calcium oxalate crystals. he leaves 
and other organs of many plants con- 
tain conspicuous crystals of calcium 
oxalate, which appears to be a waste 
product. The forms of the crystals are 
very diverse. One of the commonest 
is a compound crystal having the ap- 
pearance of a rosette and known as a 
rosette crystal (Fig. 21). Another com- 
mon form is a long, slender crystal, or 
raphide. Such crystals usually lie par- 
allel to each other in a bundle, which 
is sometimes found in a special saclike 
cell (Fig. 22). In certain cases these 
cells have tapering end walls which are thin at the apices 
(Fig. 22). When the sacs are injured mechanically, the raphides 
are shot through the thin points (Fig. 23). Certain kinds of 


Fig. 23. Ejection of raph- 
ides from saclike cell of 
Colocasiaesculentum. (x 160) 
