98 STRUCTURAL BOTANY 



purposes, and the intermediate layers also disappear, so 

 that the wall of the ripe sporangium is only one cell 

 thick. In the mean time the sporogenous tissue goes on 

 increasing, but all its cells do not become mother-cells ; 

 a good many break down and give up their contents, which 

 serve, together with the tapetum, to feed the survivors. 



The remaining mother-cells, of which there are a 

 large number in the sporangium, then divide each into 

 four, the nucleus dividing twice before the partition- 

 walls are formed. .Finally, the four daughter-cells are 

 arranged in a tetrahedron. This rule of the division of 

 the spore mother-cells into four is wonderfully constant, 

 and holds good throughout the whole of the Mosses and 

 Vascular Cryptogams, as well as in the anthers of all 

 Flowering Plants. 



The young spores of Equisetum, when first formed, 

 have a thin wall of cellulose only, but as they ripen the 

 structure becomes very complicated and characteristic. 

 The actual membrane of the spore consists of three layers, 

 but outside all these we find a structure quite peculiar 

 to Equisetum, namely, the elaters. They are formed 

 from the fourth or outermost layer of the membrane 

 the epispore, as it is called ; this layer splits along spiral 

 lines into two long bands (with flattened ends), which, 

 until the spore is mature, remain closely wrapped round 

 it (see Fig. 38, 5). When the spores are quite ripe, and 

 getting dry, however, the two elaters stretch themselves 

 out, remaining attached only in the middle of their 

 length, and at one point (Fig. 38, 7). If it is damp 

 they coil themselves up again (Fig. 38, 6). These 

 extraordinary hygroscopic movements may be repeated 

 an indefinite number of times, as we can see by mounting 

 some spores on a dry slide under the microscope, and 



