LYCOPODIUM SPOROPHYTE 285 



small tetrahedral spores : in preparing the sections, 

 however, the spores are often washed out from the 

 sporangia. 



Examine a good section of a sporangium under a high 

 power : the wall will then be seen to be of approxi- 

 mately uniform thickness throughout, and consists of 



a. A well-marked outer layer of cells of considerable 

 size. 



1). An inner ill-defined band, consisting of the remains 

 of disorganized cells. 



If the wall be observed in surface view, the cells will 

 be seen to be of sinuous outline, and somewhat 

 elongated, with the exception of a zone which indicates 

 the line of ultimate dehiscence of the sporangium : 

 here the cells are shorter, and the walls are straight. 

 The line of dehiscence may also be recognized as a 

 thinner point in the wall as seen in section. Note also 

 the structure of the spores ; they have the form of a 

 rounded tetrahedron, and the outer wall is covered with 

 peg-like projections. 



By cutting similar median longitudinal sections of cones in 

 various stages of development, and comparing them, the history 

 of development of the sporangium may be traced. It may be 

 seen that the sporangium arises as a multicellular outgrowth of 

 the upper surface of the leaf : at an early stage the archesporium 

 may be recognized in these sections as a hypodermal cell, or even 

 several cells : a careful comparison of tangential or transverse sec- 

 tions shows that there is a row of five to eight archesporial cells, 

 of which usually one only appears in a thin radial section : the 

 superficial layer of cells above it gives rise by division to three 

 layers ; of these the innermost is the tapetum, which, together 

 with the next outer layer, is disorganized as development pro- 

 ceeds, while the outermost layer is still persistent in the mature 



