GROUP IV. — PHANEROGAM lA. 



457 



In the course of frequent cross-pollination it is inevitable that 

 the pollen of other species of plants should be applied to the stigma ; 

 but while the pollen of plants of widely different species is wholly 

 without effect, that of nearly allied species, particularly those 

 belonging to the same genus in certain groups, has a fertilising 

 effect ; the result of cross-fertilisation between species is hyhridi- 

 sation, that is, the development of a plant which combines the 

 characters of both parent-species to a certain extent, and which is 

 known as a hrstard or hybrid. 

 Hybrids are for the most part 

 sterile among themselves, but 

 are often fertile when crossed 

 again with a plant of either 

 of the parent-species or of some 

 allied species. While hybrids 

 are produced with great ease 

 in certain genera, as Salix and 

 Cirsiura, in others the artificial 

 production of hybrids has never 

 yet been found possible even 

 between very closely allied 

 species, as the Apple and Pear. 



Fertilisation. As in other 

 plants, so here, the process of 

 fertilisation consists in the fu- 

 sion of the male and female 

 reproductive cells. The way in 

 which it takes place in Phane- 

 rogams is briefly as follows : — 

 The pollen-tube makes its way 

 to the female organ, entering 

 the neck of the archegonium in 

 the Gymnosperms, and coming 

 into close relation with the egg- 

 apparatus in the Angiosperms. 

 The apex of the wall of the 

 pollen-tube is at this time 

 mucilaginous : a small nu- 

 cleated mass of pi-otoplasm, the male cell (see p. 450), is forced out 

 through the mucilaginous membi*ane, into the oosphere ; the 

 nucleus of the male cell {male pronucleus) and that of the female 



Fig. 29 i. — Epipacii^t latifolia. A Longi- . 

 tudinal section through a flower-bud. B 

 Open flower after removal of the perinntb, 

 with the exception of the labelluin, I. CThe 

 reproductive organs, after the removal of 

 the perianth, seen from below and in front. 

 1)0.8 B : the point of a lead-pencil (h) is in- 

 serted after the manner of the proboscis of 

 an insect. E and F The lead-pencil with 

 the pollinia attached ;/K' ovary ; I labellnm, 

 its sac-like depression serving as a nectary ; 

 lithe broad stigma; en the connective of 

 the single fertile anther ; p pollinia ; h the 

 rostellum ; a- x the two lateral staminodes ; 

 i place where the labellum has been cut 

 off ; » the gynostemium. (Aaer Sachs.) 



