PLANT WORLD CONTINUED 



because sperm must swim to the neck-canal and pass through this into 

 the venter. Here it enters the egg and the nuclei of sperm and egg unite. 

 The fertilized egg now divides -by mitosis very rapidly, the upper 

 cells form a large globular spore-case with a thick central column within 

 known as the columella. This is surrounded by a dome of spores, around 

 which the wall of the sporangium is formed. The spore-case later 

 pushes against the wall of the archegonium by enlarging. The wall is 

 then ruptured, the top portion remaining as the calyptra ( ), 



(Fig. 119), while the spore case later opens by means of a lid. The lower 

 cells produced by the dividing oosperm becomes a swollen foot, which 

 is imbedded in the tissues below. It remains connected with the spore- 

 case by a short stalk. 



The structure which thus develops from the fertilized egg-cell is 

 called the sporophyte (Fig. 114) stage of Sphagnum. In fact, all such 

 simple plants which develop spores are called sporophytes. 



Simultaneously with the maturing of the sporophyte. the apex of 

 the female branch elongates into a leafless stalk about half an inch or 

 more in length, known as the pseudopodium. It is supposed that the 

 reason that the pseudopodium and sporophyte grow thus simultane- 



Fig. 115. 



Antheridium of Pteris (B.), showing wall Fig. 116. Sphagnum sp. 



cells (a.), opening for escape of sperm mother 



cells (e.), escaped mother cells (c.), sperms A - B - .young protonemata ; C., older pro- 

 free from mother cells (6.), showing spiral t** wlt * Iea j^ bud ,' k - r - **MI 

 and multiciliate character. (After Caldwell.) rhizoids. (After Campbell.) 



ously is probably due to the cells in the foot secreting a substance which 

 stimulates the cells in which it is imbedded to divide and enlarge, result- 

 ing finally in the formation of the pseudopodium. The advantage the 

 plant gains is that the spore-case is raised to a higher plane and it can 

 thus throw its spores much farther than would otherwise be the case. 



As Sphagnum possesses no chlorophyl, it does not manufacture its 

 own food and must therefore live on the absorption of food-matter from 

 the gamete plant through the foot. 



The spores themselves develop in the following manner. (Fig. 

 115.) In the spore-case the inner cells differentiate into two kinds, one 

 making up the larger portion of the tissues, and the other larger and 

 richer in protoplasm, forming a dome of sporogenous or spore-forming 

 tissue near the upper wall. It is from this latter type of cell that the 

 spore-mother-cells are developed. 



