174 THE STRUCTURE OF ECONOMIC PLANTS 



average of 99 per sq. mm. in the upper epidermis and ixi in the 

 lower. 



The mesophyll is chlorenchymatous and rather compact with 

 no sharp differentiation between the palisade and spongy tissue. 

 There is apparently a correlation between the thickness of the leaf 

 and the character of the mesophyll cells; and Artsch wager found 

 that in thick leaves, practically all of the cells are of the elongated 

 palisade type, while in very thin leaves, the entire mesophyll con- 

 sists of relatively short, round cells. The main veins are like those 

 of the petiole and are reinforced by zones of coUenchyma which 

 abut both epidermal layers, the more extensive abaxial zone being 

 several layers in width. 



In connection with investigations of the curly-top disease in the 

 sugar beet, Esau (7) has studied the ontogeny of the phloem in the 

 leaves. The sieve tube and its companion cell arise from the 

 mother cell by a longitudinal division. The daughter nuclei in 

 the two cells are unlike, that of the companion cell being more 

 granular and deeply staining. (Fig. 136, 4.') The cytoplasm is 

 vacuolate, the vacuoles being numerous and small in young primary 

 phloem cells, and large and relatively few in number in young 

 secondary phloem cells. (Fig. 136, 4 and 2.) As the sieve tube 

 develops, characteristic slime bodies form which are plastic and 

 shaped like irregular drops. These are regarded as inclusions of 

 proteinaceous material which later disintegrate and become a part 

 of the vacuolar substance in mature sieve tubes. A very charac- 

 teristic feature of the phloem is the development of plastids in the 

 sieve tube and companion cell. These are commonly disk-shaped, 

 but sometimes appear as thick rings with a clear area in the center. 

 The plastids persist as permanent cell organs and are at first scat- 

 tered throughout the cytoplasm, later accumulating in greater 

 numb&rs near the sieve plates. (Fig. 136, 2.) As the sieve tubes 

 mature and later become crushed, the plastids begin to disintegrate, 

 this process being initiated by swelling. 



The nucleus of the sieve tube disintegrates during its maturation 

 at about the time that the slime bodies break down. The walls of 

 the tube thicken perceptibly, especially in the primary phloem, and 

 the areas where the sieve perforations develop can be detected at 

 about the time that the slime bodies are formed. (Fig. 136, /.) 

 Later, the plate thickens, the pores become distinct, the sieve ele- 

 ment attains its full length, and the tube may be regarded as mature. 



