THE DEVELOPMENT OF ORGANISMS 



779 



tative and reproductive, and the latter bears archegonia. A male 

 gametophyte, falling or wind-borne, lands among megasporangia ; 

 it bursts from the microspore wall and sperms are liberated. One of 

 these enters an archegonium and fertilises an egg-cell. This develops 

 into a nutritive suspensor and an embryo with foot and leaves, 

 shoot and root. This embryo gets free on the surface of the strobilus, 

 very much as if it were a seed sprouting, and by and by the whole 

 strobilus falls off. The schema is: 



(A) Microspore 



(B) Megaspore 



-> Reduced male — 



gametophyte, 

 within microspore, 

 with antheridium 



-> Reduced female • 

 gametophyte, 

 within 

 megaspore, 



with 

 archegonia 



-> Biciliate 

 sperm-cell 

 fertilises 



Egg-cell 

 within 



megaspore 



Embryo 



sporophyte 



with 



microsporangia 



and 

 megasporangia 



ISOETES. — In the aquatic or amphibious quillworts of this 

 aberrant genus, the stem is extremely short, the leaves are some- 

 what grass-like, and each has a basal sporangium. This diverges in 

 the course of development to become a micro- or a mega-sporangium. 

 The spores escape by a decay of the sporange wall and the micro- 

 spore forms an interestingly reduced male gametophyte, which 

 points the way to seed plants. It consists of a single vegetative cell 

 and an antheridium with only four sperm-mother-cells (in seed 

 plants further reduced to two). The sperms are large, coiled, and 

 multiciliate as in ferns and horsetails. The female gametophyte, 

 telescoped into the megaspore, does not protrude and is accessible 

 only through a crack. From the fertilised egg-cell the sporophyte 

 develops, and shows a striking likeness to the embryo of Mono- 

 cotyledons. 



HORSETAILS. — The historian of the fern -like plants or Pterido- 

 phytes must take account of two very small living plants, Psilotum 

 and Tmesipteris, which are milestones in their way, and of the 

 extinct Carboniferous Sphenophylls; but in our merely illustrative 

 survey we must now pass to the Horsetails or Equisetaceae, a great 

 group in the past, rising to the height of a hundred feet in the 

 Carboniferous, but now represented by a single genus, Equisetum, 

 and mostly small. 



They are familiar plants in their sporophyte phase, marked by 

 subterranean axis, jointed aerial stem, whorled green branches, 

 reduced scale-like leaves, and sporangia in cone-like fruct fications. 



