16 SPORES AND THALLIDIA. 



where it remains permanently connected with the mother-plant, as happens m 

 Mosses. 



The cells composing the tissues of the cylindrical, pyriform, or spherical body 

 above referred to develop in a variety of ways. Those situated near the outer 

 surface form the wall of a receptacle, and those in the interior, which serve as 

 a filling to the receptacle, form the spores. The process of spore-formation is here 

 much the same as in Ferns. The cells of the central mass, at first united into a 

 tissue, in time become isolated; each divides into four, and the spores are ultimately 

 developed from these protoplasts. The spores are then left free in the form of 

 a fine powder within the receptacle, which is called a sporogonium. In most 

 Liverworts, a group nearly allied to the Mosses, certain other cells having a curious 

 structure are formed from the internal tissue besides the spores. These are the 

 so-called " elaters ", and they serve to scatter the spores. In a few Mosses a sort 

 of central column remains in the middle of the sporogonium in addition to the 

 spores when the whole is mature. Externally the sporogonia of Mosses differ 

 very little from the cellular bodies out of which they were developed; like them, 

 they are spherical, pear-shaped, or cylindrical as the case may be. But the part 

 which subsequently opens and liberates the spores at the proper time exhibits 

 in its more minute anatomy considerable differentiation. This subject and that 

 of the elaters mentioned above will be again referred to in the section devoted 

 to the distribution of spores. 



As with the sporangia in Ferns, so also in Mosses the sporogonia are protected 

 during development from injurious external influences, especially desiccation, and 

 are wrapped in coverings which vary considerably according to their origin. In 

 Mosses a kind of cap is usually to be seen covering the young and tender sporo- 

 gonium (see fig. 191 ^), and this structure has its origin in the fruit from which the 

 sporogenous generation (or sporophyte) has sprung, the coat of the fruit being torn 

 away and its upper part carried up in the form of a cap by the sporophyte during 

 its growth from the embryo. Later on, when the sporogonium is no longer in 

 need of protection, and the presence of a cover would be detrimental in that it 

 might prevent the spores from being scattered, the cap is cast off. 



All the spores hitherto discussed originate within a tissue, and their history 

 involves the conversion of the protoplasmic contents of each compartment of the 

 reproductive part of the tissue into a spore. A second group of spores is composed 

 of those which arise from the breaking up of the protoplasmic contents of tubular, 

 club-shaped, or spherical cells not united in tissues, and are set free from their 

 birthplaces as soon as they are formed. The cells thus constituting the mother- 

 cells of spores may, by analogy, be conveniently termed sporangia. The process 

 of formation of spores within them appears to be much simpler than in Ferns, 

 Club-mosses, Horse-tails, Mosses, and Liverworts. Speaking generally, the only 

 striking differences occurring in these cases are such as affect the number and shape 

 of the spores which escape from a sporangium. 



As described in the first volume of this work (c/. vol. i. p. 23, and Plate I a-d), 



