10 SPORES AND THALLIDIA. 



careful to avoid at the present day. When we come to the description of fruits 

 and their origin, we shall have occasion to return again to this subject. 



The places where spores originate are remarkably varied. In some plants 

 nests of cells make their appearance in the interior of an extensive tissue; in others 

 single cells are exposed on the surface. The task of spore-development devolves 

 sometimes upon a part of a green stem or leaflet. Sometimes — in plants devoid 

 of chlorophyll — upon the protoplasmic contents of a tubular structure, and some- 

 times upon the abstricted ends of hyphal filaments. The best way to arrive at an 

 idea of the extreme diversity in this respect is to classify spores in groups accord- 

 ing to their mode of origin. 



One group comprises all such spores as are formed in the cells of a tissue. 

 Amongst these are the spores of Ferns, Rhizocarps, Horse-tails, Club-mosses, and 

 the numerous kinds of Mosses and Liverworts. In one sub-group of Ferns papillae 

 spring singly from the epidermis clothing the ribs of the fronds, each papilla being 

 divided by a transverse wall into a free extremity and a stalk-cell. Both cells 

 of the papilla become partitioned so as to form bodies of tissue, and the one that 

 developes from the free terminal cell assumes an oval or spherical shape. In 

 this latter ball of tissue a tetrahedral central cell and an envelope composed of 

 several layers of cells may be distinguished. By internal partition of the central 

 cell a little cluster of cells is formed, whilst, in the meantime, the inner layer of 

 cells composing the envelope is dissolved, so that the whole now assumes the aspect 

 of a receptacle inclosing a ball of cells embedded in a fluid matrix. Each cell of 

 the cluster next divides into four compartments, and the protoplasts which 

 constitute the contents of these chambers provide themselves with membranes and 

 become disconnected upon the solution of the framework of their home. These 

 separated cells are the spores. To the naked eye they have the appearance of a 

 powdery mass. As has been said, of the cell-layers which formed the envelope of 

 the sporogenous tissue, only the inner one was dissolved; the outer layer persists 

 and constitutes a kind of capsule, to which the name of spore-case or " sporangium" 

 is applied (see figs. 189 ^^, 189 ^*, 189 ^*). A collection of sporangia of this sort is 

 called a "sorus". In the Polypodiacese — a family of Ferns to which the majority 

 of European species belong — the sori may be seen on the backs of the fronds 

 (see 189 ^). Upon the veins running through the green tissue are seated little 

 cushion-like groups of cells. Each cell in one of these cushions is capable of 

 developing into a stalked sporangium, and sometimes a single sorus consists of no 

 less than fifty such stalked sporangia. In the Cyatheacese also, which include most 

 of the Tree-ferns, the sori are developed on the under side of the fronds, but in 

 their case each is borne on a kind of peg projecting at right angles to the surface 

 of the frond. The sporangia derived from the epidermal cells of this peg are very 

 shortly stalked. An annular wall is produced from the green tissue of the frond 

 and surrounds the sporangiferous peg, which consequently stands up from the 

 middle of a cup (see figs. 189 ", 189 ", 189 ^^). 



In the delicate and graceful Hymenophyllacese — Ferns with a resemblance to 



