124 A TEXTBOOK OF GENERAL BOTANY 
tubes are lined with a layer of protoplasm, but mature sieve 
tubes do not contain any nuclei. 
The sieve tubes serve primarily for the conduction of food 
material. The holes which connect the cells of the sieve tubes 
make these tubes especially suited to the conduction of protein. 

Fre. 121. Successive stages in the formation of a resin duct in pine. (x 250) 
Companion cells. The companion cells are small cells which 
are attached to the sieve tubes. Each companion cell is the 
sister cell of a sieve-tube cell, the two being formed by the 
division of a mother cell. The exact function of the companion 
cells is more or less obscure, but since they are connected by 
sieve plates with the sieve tubes which contain protoplasm but — 
no nuclei, it would appear that the nuclei of the companion 
cells serve as nuclei for the sieve tubes. The small shaded cells 
in the phloém in Fig. 114 are companion cells. 
