18. THE FRUIT. 61 



The differentiated gametangia are of various structure. The 

 antheridium is unicellular in most of the lower plants (Green 

 Algte, except Characese ; Fucacese). In all the other cases it is 

 multicellular, and of simple structure, except in the Characese, 

 where the structure is extremely complex. In some cases (Rhizo- 

 carps) the antheridium consists almost entirely of the mother- 

 cells of the spermatozoids ; in most cases the mother-cells are sur- 

 rounded by a parietal layer of cells. The pollinodium is generally 

 unicellular. 



The oogonium is unicellular. The archegonium is generally 

 multicellular, consisting of a cellular wall investing the oosphere, 

 usually prolonged into a tubular neck ; but in most of the higher 

 plants, the archegonium is reduced to a single cell, the oosphere. 

 The archicarps and procarps are unicellular in some cases, multi- 

 cellular in others ; in most cases the organ is prolonged into a 

 filament, the trichogyne, by means of which fertilisation is effected. 

 The oogonia (except those of Peronosporacese, Saprolegniacese, and 

 Characeae) and the archegonia, open, so that their contents are in 

 direct relation with the surrounding medium ; in the procarps and 

 archicarps this is not the case. 



Further details are given in Part IV. in connexion with the 

 plants to which the various organs belong. 



(c) TJie distribution of the Sexual Organs. The male and 

 female organs are either borne by the same gametophyte, or they 

 are borne by distinct male or female gametophytes ; in the former 

 case the organism is said to be monoecious, in the latter dioecious. 



When in monoecious plants the male and female organs are both 

 present in the same sorus, as in some Algae (e.g. Fucus platy carpus, 

 Halidrys and other monoecious Fucacese) and in some Mosses, the 

 sorus is said to be bisexual or hermaphrodite, and the plant is said 

 to be monoclinous ; when they are borne in different sori on the 

 same plant (e.g. in Hepaticse generally, some Mosses), the sorus is 

 said to be unisexual, male or female as the case may be, and the 

 plant diclinous. These terms are also applied to the flowers of 

 Phanerogams. 



18. The Fruit. Although the forms of fruit occurring among 

 plants are so various in their form and in their structure, it is 

 possible to include them all in a single definition. A fruit is the 

 product of a process of growth initiated as a consequence of a sexual 

 act in structures which are not themselves immediately concerned 

 in the sexual act. 



