MOEPHOLOGY OF REPEODUCTIVE ORGANS 223 



There is no difference between the gametes of such plants, which 

 are consecjuently called isogamoiis. In certain other isogamous 

 AlgsB {fig. 486) the gametes are produced singly in each cell, and 

 are then of greater dimensions, and are not furnished with fla- 

 gella. These are not discharged into the water, as are the flagel- 

 late ones. The body arising from the fusion of the gametes of 

 isogamous plants at once secretes round itself a delicate cell- 

 wall, and is called a zygospore or zygote. 



Fig. 483. 



Fig. 484. 



■mcij- 



Fitj. 483. Megaspoi-augium (^oviile) of Pini(ss.t maturity. After Dodel Port. 

 )neg. Megaspore. end. Grametopliyte or prothallium. arch. Archegonia. 

 p.g. Pollen-graia or microspore, -which has been traDsported to the micro- 

 pyle of the ovule and has put out its protliallium, tlie polleu-tube, p.t. 

 Fig. 484. Megasporangium (ovule) of an Angiosperm. me<j. Megaspore. 

 005. Oosphere. v.t. Polleu-tube. 



In others of the Algae the gametes begin to show a greater 

 degree of differentiation. In some species of Ectocarpus, 

 though they are externally similar, certain of them come to rest 

 much earlier than the others. In Cutleria they are of two sizes ; 

 they all swim about on their being liberated from the plant, but 

 the larger soon come to rest, while the smaller maintain their 

 power of motion much longer. In this plant the gametes of 

 different dimensions are developed in receptacles or gametangia 

 of different form. The more active ones are held to be male ; 



