THE WHEAT RUST 63 



each of the large irregular cells in the basal laj^er a chain of spores is 

 produced in the way just described, excepting that the chains of cells 

 at the ven.- outside of the group remain connected with one another, 

 and form, not spores, but the outer wall of the cup. The cell unions 

 that take place at the base of the cluster cup are to be compared with 

 the sexual unions which we have studied in Spirogyra and in the bread 

 mold. The cells of the basal layer, therefore, before they unite in 

 pairs, are gametes, and the irregular cells formed by their union are 

 zygotes. An important difference, however, Ues in the fact that a 

 zygote of Spirogyra is produced by a cell union and a nuclear union, 

 but a zygote of the rust results from a cell union only. The rust zygote 

 proceeds at once to the formation of many spores, and so the chances 

 for the survival and increase of the plant are greatly improved. 



91. Spermatia. — : As we have seen, the cluster cups con- 

 taining spring spores are borne for the most part on the under 

 surface of the barberry leaf. On the upper surface of the leaf, 

 and usually just opposite a group of cluster cups, there 

 appears a yellowish spot in the midst of which are a ntimber 

 of small dark dots (Fig. 26, .4). One can see in a cross 

 section of the leaf (Fig. 27, A, C) that each dot corresponds 

 to an opening in the upper epidermis of the leaf. Before 

 this opening was made, the fungous threads formed a cushion 

 below the epidermis, such as was formed before the appear- 

 ance of the cluster cup. Then the ends of the threads grew 

 upward, making a layer of slender, vertical cells whose 

 growth finally broke the epidermis. Some of the slender 

 cells grow out through the opening so made; but most of 

 them end within the ca^-ity produced by the pushing up 

 and breaking of the epidermis, and from their ends small 

 roimded cells {spermatia) are cut off one after the other. 

 The spermatia look like spores, excepting that they are 

 much smaller ; but they never, so far as has been dis- 

 covered, develop into new plants ; and they seem to be 

 quite useless. 



and disappears. These disappearing cells are to be seen only between the younger 

 spores (that is, the lower ones in each row), and even there they are often hard to 

 find. 



