MOBPHOLOGY OF EEPBODUCTIVE ORGANS 97 



water. Such a spore is known as a zoospore. It only occurs 

 in the lowest forms of plants, the Algse and Fungi. 



The asexual cells, or gonidia, which are borne upon the 

 gametophytes of the Algae and Fungi, have the same structure 

 as spores. 



In the Vascular Cryptogams the sporophyte of some plants 

 produces one kind of spore only ; that of others gives origin to 

 two kinds, which differ from each other in size and in the form 

 of gametophyte to which they give origin. They are known 

 as microspores and macrospores or megaspores respectively. 

 According to whether one or two kinds of spore are produced, 

 the sporophyte is called Iwmosporous or heterosporous. The 

 Phanerogams are all heterosporous, and special terms are 

 employed to characterise their microspores and megaspores 

 respectively. The former are known as pollen-grains, and the 

 latter as emhryo-sacs. These terms were employed before the true 

 nattire of these structures was known ; it is convenient to use 

 them still, recognising, however, their true morphological nature. 



The Sporangia. 



A good deal of variety is found in the form of the organ 

 on or in which the spores or the gonidia originate. In unicel- 

 lular plants the whole cell may become a sporangium or gonidan- 

 gium, as in Yeast, where its contents divide into fomr parts, each 

 of which becomes surrounded by a cell- wall, and the four spores 

 become liberated by the disintegration of the original cell-wall. 

 In many Fungi there is no special gouidangium, but the gonidia 

 are abstricted from certam special cells of the surface, andfre- 

 qtiently form long chains which eventually separate into the 

 separate gonidia. In others the gonidia are solitary at' the end of 

 a filament. Some botanists regard these as gonidangia, each 

 producing a single gonidium. 



In higher plants the sporangium is not a single cell, but a 

 differentiated multicellular body. 



In the group of the Bryophyta, or Moss-like plants, there are 

 no definite sporangia, but the spores are produced in a special 

 part of the sporophyte, known as the capsule or sporogonium. 

 This must not be confused with a sporangium, as it contains 

 other structures than spores ; it is a highly complex body, and 

 forms the greater part of the sporophyte. 



In all plants above the Mosses the sporangium is a well- 

 differentiated roundish or ovoid structure, possessing a wall 

 or coat, often furnished with particular cells to secure its open- 



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