32 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 4 



thesis, and therefore must receive water from a duct and 

 transmit its sugar and proteins to bundle-sheath and sieve- 

 tube, many of them, as implied in Fig. 8, stand some 

 distance removed from the nearest veinlet. It is known, 

 however, that chlorenchyma cells can draw water, and like- 

 wise pass soluble substances, from one to another, the physical 



methods whereof we shall 

 presently consider. Now 

 the distances through 

 which this method is 

 effective must of course 

 be limited, and while no 

 exact measurements 

 appear to have been 

 made, it seems highly 

 probable that the size of 

 the ultimate areas of 

 chlorenchyma inclosed by 

 the veinlets (as noted on 

 page 18) is correlated 

 with the number of chlor- 

 enchyma cells which can 

 thus effectively obtain 

 their water, and remove 

 their sugar or proteins, through one another. 



The cells of the epidermis are rectangular in section, 

 though when viewed from the surface, they are found vari- 

 ously shaped, even to lobed and interlocked (Fig. 10). They 

 contain protoplasm, but ordinarily no chlorophyll (in the 

 higher plants) ; and their walls, as proved by chemical tests, 

 are infiltrated with a special substance called cutin, which 

 renders them waterproof. Especially characteristic of epi- 

 dermis is the fact that its continuity is unbroken except for 

 the stomata, of which a single example appears in our picture 

 (Fig. 8, also 22). Stomata, however, which provide the 

 entrance and exit for carbon dioxide and oxygen, are by no 



Fig. 10. — Typical epidermal cells, with 

 guard cells, in outline, seen from the sur- 

 face ; magnified to same scale. On the left 

 Allium, on the right Sunflower. 



