AN ENCYCLOPEDIA OF HORTICULTURE. 



229 



Froterandrons and Protandrous continued, 

 ripe before the stigma of the flower is ready to receive 

 it. This is one of the most frequent adaptations to 

 favour cross-fertilisation in plants, since the pollen of a 

 Proterandrous flower is frequently removed by wind or 

 insects before the stigma is ready for its reception; 

 hence, pollen must be brought to the stigma from a 

 younger flower. 



PROTEROGYNOUS and PROTOGYNOUS 

 (from proteros, sooner, and gyne, a female). Terms used, 

 by Delpino and by Hildebrand respectively, to denote a 

 hermaphrodite flower in which the stigma is ready to 

 receive pollen before that in the same flower is ripe. 

 In Proterogynous flowers, the stigma is pollinated from 

 an older flower, and may be withered, or may have fallen 

 off, before the anthers in its own flower have burst to 

 shed the pollen. Proterogyny is not frequent. The 

 common Pellitory (Parietaria officinalis) affords an 

 example. 



PROTHALLUS, or PROTHALLIUM (from the 

 Greek words prt>, instead of, and thallos, a branch ; in 

 reference to the structure thus named taking the place of 

 a stem). The body which is developed from a spore of a 

 Fern, Horsetail, Club-moss, or Pillwort. It varies much, in 

 these four groups, in its degree of development, and in its 

 form ; but, in all cases, it is entirely cellular throughout 

 its existence, and on it are formed the organs for 

 sexual reproduction. In Ferns, the male and the female 

 organs are present in the same Prothallus ; in most 

 plants of the other groups, the Prothallus bears only 

 male or only female organs. There is a distinct alter- 

 nation of generations in plants that produce a Pro- 

 thallus, i.e., in the higher Cryptogams. The spore pro- 



FIG. 290. Youxo PROTHALLUS, much enlarged #, Prothallus; 

 rh, Root-hairs of Prothallus ; *, Spore. 



dnces the Prothallns (see Fig. 290), on which are formed 

 the sexual organs, the male being the antheridia, with 

 antherozoa, and the female being the archegonia, in 

 each of which lies the oosphere, which, fertilised by 

 antherozoa, becomes the oospore. This develops into 

 the Fern-plant bearing the well-known fronds, on the 

 back of which are visible the groups (son) of minute, 

 brown spore-cases (sporangia), in which lie numerous 

 spores, like the one with which the cycle began. The 



FIG. 291. LOWER SURFACE OF MATURE PROTHALLUS, much 

 magnified a, Archegonia ; rh, Root-hairs. 



spores are formed by vegetative growth, not by sexual 

 reproduction. It will thus be seen that the Prothallus 

 and the leafy Fern-plant are two generations in the 

 course of a single cycle. The Prothallus of Ferns 

 (except in the Moonwort group) is a flattened, green, 

 expanded body, which grows in damp places, e.g., on 

 damp bricks. It is thin, and consists of a single layer 



Prothallus, or Prothallimn continued. 

 of cells, except in the middle, where it reaches a thick- 

 ness of several layers. The cells contain an abundant 

 supply of chlorophyll bodies, which give the Prothallus 

 its colour. In general outline, the Prothallus, when full- 

 grown, usually resembles the conventional figure of a 

 heart, having one end narrowed, and a notch in the 

 broader margin (see Fig. 291). It seldom exceeds in. 

 in breadth. On the lower surface are numerous root- 

 hairs, each made up of a row of cells. On the lower 

 surface also, and along the edges, are formed the 

 antheridia. The archegonia are situated in the middle 

 of the lower surface. The antheridia originate as out- 

 growths of cells of the epidermis ; each outgrowth is cut 

 off, as a new cell, by a cell walL In some Prothalli, 

 the contents of the cell thus formed break up into a 

 number of small, rounded cells, called "parent cells." 

 In each of these there is formed an antherozoid, slender, 

 but coiled spirally in two or three turns, and provided 



Fio. 292. ANTHEROZOID, much magnified. 



with a tuft of fine hairs, or cilia, at one end (see Fig. 

 292). In most Prothalli, however, a process of cell- 

 division goes on in the young antheridinm, whereby 

 it is finally made up of a layer of cells surrounding 



FIG. 293. YOUNG ANTHERIDIUM, much magnified-*:, Central Cell, 

 filled with Parent Cells of Antherozoids ; e, e, Epidermis of 

 Prothallus. 



a central cell (see Fig. 293), and, in this latter, the 

 parent cells are developed, and produce antherozoids. 

 The outer coat, formed by the layer of cells, has to aid 

 in expelling the antherozoids when ripe, and the cells 

 do this by absorbing water rapidly, swelling, and com- 

 pressing the contents of the central cell till its apex, 



FIG. 294. RIPE ANTHEUIDIUM (much magnified), from which 

 Antherozoids have been shed by opening at o c, Empty 

 Central Cell; , e, Epidermis. 



which is not covered by the layer, is burst (see Fig. 

 294), and the "parent cells" are expelled, and, soon 

 rupturing, set free the antherozoids. The latter move 

 about actively in water, e.g., in a drop of dew or of 

 rain. 



The archegonia are situated on the lower surface, 

 behind the notch already mentioned (see Fig. 291). 

 Each originates, like the antheridia, from a cell of the 

 epidermis, from which it grows out, in a hemispherical 

 form. A cell wall forms, and cuts it off from the cell 

 of the epidermis. It increases in size, and becomes 

 further divided into three layers ; and these are again 



