100 



ANCIENT PLANTS 



stone encasing the endosperm, which is large, green, and 

 starchy, and contains the embryo with two cotyledons. 

 This embryo is small compared with the endosperm, 

 cf. fig. 57, p. 76, which is somewhat similar to that of 

 Ginkgo in this stage. 



Of the microscopic characters of the reproductive 

 organs the most remarkable is the male cell. This is 

 not a passive nucleus, as in the plants hitherto con- 

 sidered, but is an actively swimming cell of some size, 



provided with a spiral of cilia 

 (hairlike structures) whose 

 movements propel it through 

 the water. In the cavity of 

 the unripe seed these swim 

 towards the female cell, and 

 actively penetrate it. The 

 arrangements of the seed are 

 diagram matically shown in 

 fig. 68, which should be com- 

 pared with that of Cycas, fig. 

 76, with which it has many 

 points in common. 



The nature of the male 

 cell in Cordaites is not yet 

 known, but there is reason 

 to suspect it may have been 

 actively swimming also. As 

 this is uncertain, however, 



sc, 



Fig. 68. Section through Seed of 

 Ginkgo 



p.c, Pollen chamber in the nucellus n, 

 which is fused to the coat c to the level 

 /; sc, stony layer in coat; s, the big 

 spore, filled with endosperm tissue (in 

 this case green in colour) ; e, egg cells, 

 one of which will produce the embryo 

 after fertilization. 



we may consider Ginkgo the most highly organized plant 

 which has such a primitive feature, a feature which is a 

 bond of union between it and the ferns, and which, when 

 it was discovered about a dozen years ago, caused a 

 considerable sensation in the botanical world. 



To turn now to the fossil records of this family. 

 Leaf impressions of Ginkgo are found in rocks of nearly 

 all ages back even to the Upper Palaeozoic. They show 

 a considerable variety of form, and it is certain that they 

 do not all belong to the same species as the living plant, 



