98 MANUAL OF BOTANY 



ing when ripe. In the interior is a cavity, in which the spores 

 usually lie free and present the structure already described. 



The position of the sporangia varies also. Usually they arise 

 on leaves, which are then known as sporophylls, each of which 

 may bear one, two, four, or many. They may be on the back of 

 the leaf, as in most Ferns ; on its edges, as in the case of the 

 megasporangia of most Phanerogams ; or in its axil, as in many 

 Lycopodinse.. In some cases they arise on the axis or stem, as 

 the megasporangium of the Yew-tree among Gymnosperms, and 

 as in some Angiosperms. The megasporangia are more fre- 

 quently borne on the axis than are the microsporangia. 



In many cases the sporangia are found in isolated groups, 

 when each group constitutes a soriis, as in most Ferns. Some 

 times the sporangia in a sorus are not isolated but coherent 

 together, as in Marattia, and in the anthers of the Phanerogams, 



When a plant bears both microspores and megaspores, spo 

 rangia producing each kind may be present in the same sorus 

 or a sorus may consist of either micro- or mega- sporangia, 

 Generally, though not always, the sorus is covered and pro 

 teoted by an outgrowth of the surface of the plant, known as an 

 indusium. The sorus is represented in the Phanerogams most 

 clearly in the arrangement of the microsporangia of some Cycads, 

 which are collected into groups on the surface of a modified leaf. 

 A kind of indusium is present in the Cupressinese, where the 

 sporangia when young are covered by an outgrowth of the under 

 surface of the sporophyll. 



In other cases the sporangia are single and not grouped. 

 Such sporangia then represent sori, and each may be covered by 

 an indusium, as in the megasporangium of Anolla. In the 

 megasporangium of the Phanerogams, generally called the ovule, 

 some botanists hold that the coats or integuments, which invest 

 it closely, represent this indusium. 



When the spores are mature they are liberated by the burst- 

 ing of the sporangium wall, and they give rise to their gameto- 

 phytes after this liberation, the latter thus being free from the 

 sporophyte. As already noted, however, an exception to this is 

 found in the case of the megaspore of the Phanerogams, which 

 germinates in situ- in the sporangium (ovule). 



The megasporangium of the Phanerogams shows consider- 

 able diversity of form in different plants. It will be convenient 

 to reserve a discussion of its principal features till the end of the 

 present chapter, where it will be treated of under its popular 

 name ovule. 



