8 4 



Type IV. Lateral Guvs: Gvrophora DiUcnii. These are other 

 special mechanical adaptations met with in foliose thalli and are 

 typically developed only in the above mentioned species. They con- 

 sist of a cortical mechanical tissue extending from a central pillar to 

 different points of the thallus, and their action is the same as that of the 

 stays of an umbrella. The structure is due to a proliferation of the 

 lower cortical layer and like that tissue consists of dark-colored cor- 

 tical cells which do not, however, form a continuous layer ; it is 

 made up of four or five thin layers, one above the other, which are 

 connected by vertical septa of cortical tissue (to prevent breaking 

 due to lateral forces). There is little doubt that the prime function 

 of this tissue is mechanical. 



Type V. Cap with Basal Ring: Gyrophora pustulata. Another 

 highly specialized mechanical tissue is typically developed in 

 Gyrophora pustulata. It consists of pustular elevations of the 

 thallus, each one of which bears a basal ring of hyphal tissue. The 

 pustules themselves constitute a mechanical structure formed by the 

 cortical layers with the intervening filling material. This cap would, 

 however, tear easily at the margin if it were not for the supporting 

 ring. On an examination of this lichen it will be found that the 

 largest pustules or caps occur where the greatest strain is exerted, 

 that is about midway between the margin and center ; on either side 

 of this point they begin to decrease in size. The central portion of 

 the thallus is attached to the substratum by the enormous umbilicus. 



Mechanical adaptations belonging to type V. are also well de- 

 veloped in some of the Collemaceae. Under this type must also be 

 included the foldings and pustules of thalli without mechanical rings 

 as they evidently serve to resist lateral tearing forces ; such fold- 

 ings are well marked in some Stictas. In connection w r ith the me- 

 chanical function of the pustules it must not be forgotten that they also 

 increase the assimilative function by increasing the algal surface. 



There are various modifications of the types here explained ; 

 the explanations here given are sufficient to enable the student to 

 comprehend the significance of the mechanical adaptations met w r ith 

 in lichens. 



VII. THE CHEMISTRY OF LICHENS. 



There is considerable chemical activity accompanying the proc- 

 ess of assimilation, going on in the lichen. Under the influence of 

 sunlight, moisture, and a certain degree of warmth, the plant ap- 

 propriates the CO., of the atmosphere and in return gives off O. In 



