1903.] on the Origin of Seed-bearing Plants. 339 



distended with water, bursts and sets free the sluggishly moving 

 spermatozoids, which by aid of the water discharged from the pollen- 

 tubes, are able to swim to the egg-cells and effect fertilisation. 



This remarkable process, first discovered in 1896 by two Japanese 

 botanists, Ikeno and Hirase, and independently in 1897 by the 

 American Webber, occurs not only in the Cycads but also in that 

 strange plaut the Maidenhair-tree, Ginkgo, a form now completely 

 isolated, certainly rare in a wild state, and said to have been only 

 saved from extinction by cultivation around Buddhist temples in 

 China and Japan, but which has a long geological history. 



The Cycadean method of fertilisation holds exactly the middle 

 place between the purely cryptogamic process, where the active male 

 cells accomplish the whole journey to the egg by their own exertions, 

 and the method typical of Seed-plants, where these cells are little 

 more than mere passengers carried along by the growth of the pollen- 

 tube. 



The adaptations, which in the Cycads allow of pollination and 

 fertilisation on the plant, are chiefly three. 



1. The enveloj)e of the seed with its narrow opening down which 

 the pollen-grains are guided. 



2. The pollen-chamber below, in which they are received. 



3. The pollen-tube, which however plays a somewhat less im- 

 portant part here than in the higher Flowering plants, and which in 

 the Palaeozoic allies of the Cycads may perhaps have been dispensed 

 with altogether. 



There are, however, other points in which the ovule of a Cycad 

 differs from the spore-sac of a Cryptogam. Not only is the 

 megaspore solitary — that is a condition already reached among the 

 Water- ferns — but it is firmly imbedded in the surrounding tissue. It 

 is no longer a mere spore destined to be shed, but remains throughout 

 an integral part of the ovule, while the ovule ripens into a seed and 

 ultimately germinates. Thus the whole development of the prothallus 

 takes place within the seed, and this requires special methods of food- 

 supply, involving a complexity of structure far beyond that of any 

 cryptogamic spore-sac. When the time for dispersal comes the seed 

 is shed as a whole. 



There is, however, another character commonly regarded as 

 essential to the definition of a seed : a seed should contain an embryo ; 

 this implies, that after the egg-cell has been fertilised, the young 

 plant develops to a certain extent while still within the seed and 

 before it is shed. In the ripe seed the embryo passes into a resting 

 stage and only resumes its development when the seed begins to 

 germinate and the embryo becomes a seedling. Usually too, the 

 ripening of the seed itself is dependent on the development of the 

 embryo ; if there is no fertilisation, there is no true seed, only an 

 abortive ovule. 



In the Cycads this is not the case ; the ovule ripens into a full- 

 sized and apparently normal seed, even if fertilisation has failed. In 



