6i6 THE STRUCTURE OF ECONOMIC PLANTS 



of the sieve tube in conjunction with its obliteration or partial 

 collapse. 



On the basis of the structural characteristics of the sieve tubes, 

 Crafts proposed that the movement of substances might take place 

 "partly through sieve tube lumina and partly through phloem 

 walls," but the subject of translocation is a controversial one and 

 physiologists are by no means in agreement on this point. In 

 this connection, the role of the phloem parenchyma in transloca- 

 tion has been generally ignored, and it seems very probable that 

 a considerable proportion of the materials translocated in plants 

 is moved through parenchymatous tissues. 



The Leaf. — The leaves are relatively thin as compared with 

 other cucurbits. The radial walls of the upper epidermal cells 

 are straight, while those of the lower epidermis are sinuous in 

 outline. The stomatal frequency in C. Pepo is about 150 per 

 sq. mm. for the upper surface as compared with about 350 on 

 the lower one. Hairs are also more abundant on the lower surface 

 than on the upper in a ratio of approximately four to one, there 

 being about 80 per sq. mm. on the former. According to Yasuda 

 (41), the lower epidermis in C. Pepo may be two or three layered 

 in some places; and at these points, the leaf has an etiolated appear- 

 ance. The palisade region comprises about three-fifths of the 

 thickness of the blade, consisting in some parts of a double row 

 of cells, and there are from two to six layers of loosely organized 

 isodiametric parenchymatous cells in the spongy region. 



The subterete petiole is slightly grooved on its adaxial surface 

 and the center is hollow throughout. A limited number of 

 stomata occurs in the epidermis, the frequency being approximately 

 II per sq. mm.; and trichomes develop as in the blade. Strands 

 of collenchyma occur outside the ring of fibrovascular bundles 

 which are arranged in pairs, except for the largest one which 

 extends along the abaxial side of the petiole. 



The Tendril. — As described by van Tieghem (37), the branched 

 tendril of the cucurbits is a shoot, the terminal portion being a 

 specialized leaf arising on a branch. This appears to be an axillary 

 branch, but it is actually a terminal structure that is laterally 

 displaced owing to the sympodial system of branching. Lisk (x6) 

 has pointed out that, although the physiology of the tendril has 

 received exhaustive study, there has been relatively little work 

 done on its anatomical structure. In C. Pepo, the proximal portion 



