16. ASEXUAL REPRODUCTIVE ORGANS. 51 



termed cilia. Motile spores are termed zoosporcs. Ciliated zoo- 

 spores are common among the Algae, and they occur in some Fungi. 



The spores produced asexually by the sporophyte of any one 

 plant are commonly of one kind only ; when this is the case the 

 plant is said to be homosporous. But in some of the Pteridophyta, 

 and in all Phanerogams, which are therefore said to be heterospor- 

 ous, there are two kinds of asexually produced spores, which differ 

 in size and in the nature of the organisms to which they respec- 

 tively give rise, and are distinguished as microspores and macro- 

 spores. In the Phanerogams, the microspores are commonly termed 

 pollen-grains ; and the macrospores, embryo-sacs. 



16. General Morphology of the Asexual Reproduc- 

 tive Organs. In the great majority of plants the asexual pro- 

 duction of spores takes place in the interior of an organ termed a 

 sporangium. 



Whilst in some plants the asexual production of spores is not 

 limited to any particular portion of the body, in others it is so 

 limited. When this is the case, the portion of the body which 

 performs this function differs more or less widely in form from 

 the vegetative portions, and it is distinguished as the sporophore. 

 When the body is differentiated into root and shoot, the sporophore 

 is always part of the shoot. 



In those plants in which the shoot is differentiated into stem 

 and leaf, the development of spores is commonly confined to the 

 leaves. A leaf bearing one or more sporangia is termed a sporo- 

 phyll. 



(a) The Sporangium. In unicellular plants (e.g. Yeast, Haema- 

 tococcus) the cell, that is the whole body of the organism, becomes 

 one sporangium. In this case the development of spores closes the 

 life of the organism, for the protoplasm is used in the formation 

 of the spores, and the cell-wall is ruptured to allow of their escape. 



In simple multicellar plants ( e.g. Ulva, Ulothrix) each cell 

 eventually acts as a sporangium, giving rise to spores. W T ith the 

 formation of spores the life of each cell is closed ; so that when all 

 the cells have formed spores the life of the organism is ended. 



In plants of higher organization the formation of spores is 

 limited to certain cells, so that the formation of spores no longer 

 necessarily puts a term to the life of the organism. It is in these 

 plants that distinct sporangia are to be found. 



In the Algae and Fungi, the sporangium, when present, usually 

 consists of a single cell. In all plants higher than the Algae and 



