78 Plants and their Ways in South Africa 



the younger portions. In a few cases, however, for example 

 Aloe, Yucca, Dracaena, Aristea corymbosa, Benth., the pericycle 

 remains meristematic and continues to lay down bundles and 

 ground tissue by which the bundles are surrounded. These 

 secondary bundles are confined to the stem (cauline) and are 

 without the protoxylem of the common bundles. 



f 



FIG. 75. Transverse section of Z^a mays. The cortex and pith are not 

 clearly defined in the ground tissue. Each bundle is surrounded by a bundle 

 sheath and contains two large vessels. An air cavity is formed by the breaking 

 down of cells in the region of the protoxylem. 



Phloem in dicotyledons may comprise sieve tubes, with smaller com- 

 panion cells adjacent to them, phloem parenchyma and sometimes fibres. 

 In monocotyledons only sieve tubes and companion cells are present, 

 while in gymnosperms the sieve tubes have no companion cells. 1 



The secondary wood of gymnosperms with few exceptions consists of 

 tracheids with bordered pits. In angiosperms the wood is marked by con- 

 spicuous vessels and wood parenchyma is present. Tracheids on the 

 other hand are more rarely present. 



Root Structure. While a carrot is not a typical root it 

 is easily cut, and something of its structure may be made out 

 with a hand lens or even with the unaided eye. It is a large 

 conical tap root tapering to a slender point with secondary 



1 A companion cell is a cell much smaller in diameter and companion 

 to a sieve tube. It is possible that their nuclei assist the activity of the 

 sieve tubes, which, although they act as living cells, strangely enough lose 

 their nuclei. 



