64 



THE GARDENER'S ASSISTANT. 



the plethora induced by over-nutrition being too 

 great, but by judiciously starving the plants 

 producing such " perfect double " flowers, the 

 sexual elements become strengthened, and semi- 



double or almost perfectly single flowers result, 

 which, when fertilized with pollen from petaloid 

 stamens, yield seeds again capable of producing 

 double-flowered plants. 



Fig. 81. —Life-history of a Fern. 



l A Feru-prothallium seen from the under side. - Ah archegoniura in longitudinal section. 3 An antheridium. ■» Escape of spermatozoids 

 from antheridium. 5 Young sporophyte with first leaf arising from the prothallium. 6 Complete sporophyte of Asplenium Ruta-muraria. 

 " Under surface of pinnule of same, showing linear Bori and lateral indusia. 8 A young prothallium arising from a spore; the spore is below. 

 natural size; l x 8; 2, 3, 4 x 350 ; 5 x 6; " x 3; * x 240. (Kerner.) 



Cryptogamic or Flowerless Plants, Ferns. 



In the case of cryptogamic or flowerless plants, 

 a kind of fertilization takes place, and it is quite 

 possible to obtain what are analogous to cross 

 breeds and hybrids amongst Ferns for example. 

 These so-called flowerless plants are naturally 

 increased by spores. Spores are asexual bodies, 

 borne in little cases, that are arranged in clusters 

 or lines on the backs of fern-fronds, &c, and 

 take the place of seeds, though in structure and 

 function they are really different things. The 



spore-cases are hygrometrical, and burst open 

 when the spores are fully developed; and these, 

 being very small and light, are wafted about by 

 the faintest breeze, and grow upon wet rocks, 

 tree-trunks, walls, or soil, almost anywhere. 

 No cotyledons, or seed-leaves, are produced 

 as in seeds, but the spore merely throws out a 

 dark-green flattish growth of cellular tissue, 

 somewhat horseshoe-shaped, and called a pro- 

 thallus or first expansion. Spores are merely 

 vegetative bodies, and are not formed directly 

 by the sexual union of male and female parents, 



