238 



ELEMENTARY STUDIES IN BOTANY 



show a series of thickened rings, and are called annular 

 vessels (Fig. 207, B) ; while others, and among them the 

 largest, have numerous thin spots in their walls which look 

 like dots of various sizes, and these are the doited or pitted 

 vessels, often called dotted ducts (Fig. 207, C). These pitted 

 vessels are often very large, their openings being visible to 

 the naked eye in the cross-section of oak wood. 



FIG. 207. Vessels: A, spiral vessels; B, annular vessels; C, pitted vessel ("dotted 

 duct ") ; D, sieve vessel ; E, a sieve-plate. A and B after BONNIER and SABLON, 

 C after DEBARY, and D after STRASBURGER. 



The cells of the bastjthat conduct prepared food are called 

 sieve vessels, TSecause in their walls, usually the end walls, 

 there appear areas full of perforations, like the lid of a pepper- 

 box, these areas being called sieve-plates (Fig. 207, D and E). 



A prominent feature of such stems is that they can in- 

 crease in diameter. If the stem lasts only one growing 

 season, that is, if it is an annual, there is no increase in di- 

 ameter ; but if it lasts through several seasons, that is, if it 

 is a perennial, it increases in diameter from year to year. 

 Naturally, annual stems belong to herbs and perennial 



