272 



POTJNDATIONS OF BOTANY 



330. Examination of the Thallus. — The thallus of Physcia as seen 

 in cross-section will be found to consist of four layers, the upper 

 cortical, gonidial, mediillary, and the lower cortical. The cortical 

 layers wiU be seen to serve for protection, answering the purpose of 

 an epidermis or bark. The cells which compose them make what 

 is called a false parenchyma, — resembling parenchyma in form but 



as to origin being trans- 



hyphse composing the 

 medullary layer. Are 

 there any cross-parti- 

 tions? Do any cells 

 appear circular, and if 

 so, what is the explanar 

 tion? The upper por- 

 tion of the cortical 

 layer, having green 

 cells intermixed, con- 

 stitutes the gonidial 

 layer. Why should the 

 green cells be at the 

 upper part of the med- 

 ullary layer ? Can you 

 detect any connection 

 between the green cells 

 and the hyphse? Do 

 these green cells re- 

 semble any cells pre- 

 viously studied ? 

 Make a diagram to show the structure of the thallus. 

 What arrangement of layers would you expect to find in a lichen 

 thallus, upright or suspended ? Compare the arrangement in the 

 fruit-body (apothecium), describe, and sketch.. How does the layer 

 of cells beneath the spore-sacs resemble the cortical layer ? AH but 

 these two layers may be considered as part of the thallus. To make 

 out the details of the fruit, the section must be very thin. 



Fi&. 200. — Transverse Section through Thallus 

 of a Lichen (Stlctafuligmosa). (x BOO.) 



c, cortical or spidermal layer ; g, gonidia ; h, hyphse. 



