THE TISSUE SYSTEMS n 



features for comparison which may be utilized in the study of 

 evolution, and because their conservatism makes them on the whole 

 the least variable of the elements entering into the composition of 

 the higher plants. Likewise, by reason of their resistance to decay, 

 they are more likely to be preserved as fossils. Beyond emphasiz- 

 ing the complexity of the fibrovascular system, it need not be 

 further considered at the present time. 



FiG. 8. Transverse section of the leaf of the white pine (Pinus Strobus), showing 

 the three tissue systems in a leaf. 



We may next consider the tissue arrangements in another of 

 the plant organs, namely, the leaf. In the figure (Fig. 8) is repre- 

 sented the transverse section of the needle of Pinus strobus. Ex- 

 ternally the epidermis forms a boundary of a single row of cells, 

 continuous except where interrupted by the occurrence of stomatic 

 openings. Beneath the epidermis lies a layer which is ordinarily 

 known as the hypoderma and is more strongly developed in fossil 



