THE TISSUES OF THE STEM 



47 



the superficial tissues may be peeled off, separating with special ease 

 in the spring. The line of easy rupture is the cambium itself, and 

 the reason why it splits so easily is that in the spring it is actively 

 growing, and its cells are then specially thin-walled and weak. It 

 will be necessary first to examine this tissue in detail, since it 

 produces such important changes. 



FIG. 33. 



Transverse section of a single vascular strand from the young stem of the Elm. 

 =endodermis. sc = sclerotic pericycle. s<= sieve- tubes. tc = cambium. />..= 

 pitted vessels. />*=protoxylem. w=medullary rays. ( x 150.) 



In herbaceous stems of Dicotyledons, such as the Sunflower, Fig- 

 wort, or Cucumber (Figs. 23, 25), the cambium is seen between the 

 wood and bast of the vascular strands. In woody stems it occupies 

 a similar position, as in the Elm (Fig. 33). The difference is only 

 one ot the proportion of the tissues, and of the activity of the 

 cambium. Where the strands are separate, as in herbaceous stems, 

 the cambial activity may be seen to bridge over the spaces between 

 the strands, thus completing the cambial ring. The cambium thus 

 forms a complete cylinder. The parts within the strands are called 



