1 88 REPRODUCTION 



spore which gradually grew out into delicate hyphae. All these spores 

 contain fat globules and finely granular protoplasm with a very large number 

 of minute nuclei; the presence of the latter has been demonstrated by 

 Haberlandt 1 and later by Zopf 2 who reckoned about 200 to 300 in the 

 spore of Mycoblastus sanguinarius. These nuclei had continued to multiply 

 during the ripening of the spore while it was still contained in the ascus 8 . 

 Owing to the presence of the large fat globules the plasma is confined to 

 an external layer close to the spore wall ; the nuclei are embedded in the 

 plasma and are connected by strands of protoplasm. The epispore in some 

 of these large spores is extremely developed: in some Pertusariae it 

 measures 4-5 /* in thickness. 



/ POLARIBILOCULAR SPORES. The most peculiar of all lichen spores 

 are those termed polaribilocular signifying a two-celled spore of which the 

 median septum has become so thickened that the cell-cavities with their 

 contents are relegated to the two poles of the spore, an open canal frequently 

 connecting the two cell-spaces (Fig. 107). Other terms have been suggested 



and used by various writers to describe this unusual 

 character such as blasteniospore 8 , orculiform 4 and 

 placodiomorph 5 or more simply polarilocular. 



The polarilocular colourless spore is found in 

 a connected series of lichens crustaceous, foliose 

 and fruticose (Placodium, Xanthorza, Teloschistes). 

 In another series with a darker thallus (Rinodina 

 and Physcia) the spore is brown-coloured, and the 

 Fig. 107. Polarilocular spores, median septum cuts across the plasma-connection. 



a. Xanthoria parietina Th. T . , .... ., 



Fr. ; b, Kinodfnaroborisl'h.. ^ n other respects the brown spore is similar to the 

 Fr. ; f, Physdapulverulenta colourless one and possesses a thickened wall with 



Nyl.; d, Physcia ctltaris DC. 



x6oo. reduced cell-cavities. 



The method of cell-division in these spores resembles that known as 

 " cleavage by constriction," in which the cross wall arises by an ingrowth 

 from all sides of the cell ; in time the centre is reached and the wall is com- 

 plete, or an open pore is left between the divided cells. Cell " cleavage " 

 occurs frequently among Thallophytes, though it is unknown among the 

 higher plants. Among Algae it is the normal form of cell-division in Clado- 

 phora and also in Spirogyra, though in the latter the wall passes right across 

 and cuts through the connecting plasma threads. Harper 6 found "cleavage 

 by constriction " in two instances among fungi: the conidia of Erysiphe and 

 the gametes of Sporodinia are cut off by a septum which originates as a 

 circular ingrowth of the outer wall, comparable, he considers, with the cell- 

 division of Cladophora. 



1 Haberlandt 1887. * Zopf 1905. a Massalongo 1852. 4 Koerber 1855. 



5 Wainio i. 1890, p. 113. 6 Harper 1899. 



