2 PHYSIOLOGICAL SERIES. 



A. 1. A Red Sea-weed (Delesseria sanguined). The perennial 

 stem-like portion of the thallns has its cells crowded with 

 reserve food-material in the form of aleurone grains. It 

 gives origin in the spring to leaf-like structures that die off 

 in the autumn. Each has an axial bundle of somewhat 

 elongated cells from which other bundles diverge and 

 branch. They are thought to give some extra strength to 

 this portion of the thallus, and serve as special paths for 

 conduction of nutriment. The resemblance to the " veins " 

 (vascular bundles) in true leaves is very great. 



Potter, J. Marine Biol. Assoc., vol. i. 1889, p. 171. 



A. 2. Transverse section of the stem of a Tree-fern (Cyathea 

 dregei). On the inner and outer side of each vascular 

 bundle is a flattened bundle of almost black, greatly elongated 

 (prosenchymatous), thick-walled cells derived from the 

 fundamental tissue ( = tissue between epidermis on the 

 outside and vascular bundles in the interior). The in- 

 creased thickness of the walls of these cells is due to a 



Fig. 1. 



c.s. 



X 110. 



secondary deposit of lignone, which renders them hard and 

 stiff. This form of tissue is known as " Sclerenchyma " ; it 

 gives strength and stiffness to the stem. The drawing 

 shows the structure of the sclerenchyina. (Fig. 1.) 



