66 Plants and their Ways in South Africa 



protoplasm ; they have markings of different patterns due to 

 successive thickening layers which are laid down on the inner 

 surface of the walls and serve to strengthen them. Vessels 

 convey the transpiration current with its dissolved food 

 material in its upward flow and at length they may serve only 

 as air carriers. Because of the appearance given by their 

 markings vessels are known as trachem. 



Fibres may have the same markings as vessels ; they are 

 then called tracheids. 



Vessels and tracheids are found in the wood of plants. 

 The first vessels to be formed (protoxylem) have the thicken- 

 ing laid down in rings, or spirals. Those formed later on have 

 simple or bordered pits and ladderlike or scalariform markings. 



Sieve tubes are found outside the cambium in the phloem. ^ 

 Their end walls are not wholly absorbed but are perforated in a 

 sieve-like manner. They have not the markings of vessels 

 and retain their protoplasm as long as they perform their 

 function. There is reason to believe that they serve to convey 

 rapidly food containing nitrogen from the place where it is 

 elaborated to parts below. 



Besides wood vessels and sieve tubes there are latex 

 vessels or milk tubes. They branch and fuse, forming a 

 network. The walls remain cellulose and are but little if at all 

 thickened. Like the sieve tubes they are without markings. 

 Their contents, latex, are milky, e.g. Asclepias, Euphorbia^ 

 Carissa, Sonchus ; watery in the poppy family (Papaveraceae), 

 where coloured latex is also found. Latex contains nutritive 

 substance and possibly waste material. 



A variety of other substances are found within cells either 

 as food or as waste products. Of these may be mentioned : — 



I. Starch. — This is one of the commonest forms in which 

 reserve carbohydrates may be found. In green parts it is 

 formed as very small grains within chloroplasts. In storage 

 organs, however, as in seeds and tubers, they are formed on 

 leucoplasts and are large and conspicuous. A starch grain 

 shows an inner watery spot, the hilum, with layers around it 



^ See p. 72. In some stems sieve tubes are also internal to the wood. 

 Some of these are mentioned farther on in the text. 



