THE TISSUES OF THE STEM 



4i 



so to construct the vascular skeleton of the shoot. Such a skeleton 

 is shown for Clematis in Fig. 20. Here the leaves are arranged in 

 pairs, and from each leaf three vascular strands pass into the stem. 



Each strand curves downwards before 

 reaching the centre of the stem, and 

 after taking a straight downward 

 course to the level of the next pair 

 of leaves it forks. The two shanks 

 are then inserted right and left upon 

 the strands that enter there. If a 

 transverse section be cut at any 

 point between the pairs of leaves, 

 the section shows six main strands 

 arranged in a ring, with smaller 

 strands between them (Fig. 21). A 

 very simple connected system of 



Fig. 20. 



Clematis viticella. End of a branch 

 which has been made transparent by 

 the removal of the superficial tissues 

 and treatment with caustic potash. The 

 emerging strands have been slightly dis- 

 placed by gentle pressure. Ihe two 

 uppermost pairs of young leaves (W 1 , bl 2 ) 

 are still without leaf -traces; v, apical 

 cone. (S. after Naegeli.) 



F03. 



Fig. 21. 



Transverse section of an internode of a stem of 

 Clematis, showing a ring of six larger and six smaller 

 vascular strands, surrounding the central pith, and 

 covered externally bv the thick cortex. In the 

 adjoining skeleton it is only the six larger strands 

 that are represented. The collenchyma massed at 

 the projecting angles is dotted. ( x 15.) 



vascular strands is thus formed, and it illustrates the arrangement 

 usual for them in the shoots of Dicotyledons. There are differences 

 in the number of the strands entering from each leaf in various 

 examples. In some there is only one strand, in others more. The 



