AN ENCYCLOPADIA 
OF HORTICULTURE. 229 
Proterandrous 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. F 
PROTHALLUS, or PROTHALLIUM (from the 
Greek words pro, 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- 
Fic. 290. Younc PROTHALLUS, much enlar; , Prothallus ; 
rh, Root-hairs of Prothallus; s, Spore. : 
duces the Prothallus (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 (sori) of minute, 
brown spore-cases (sporangia), in which lie numerous 
spores, like the one with which the cycle began. The 
Fic. 291. Lower SURFACE OF MATURE PROTHALLUs, much 
magnified. ia ; rh, Root-hairs. 
—a, 
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 — 
The Prothallus of Ferns 
course of a_ single cycle. 
(except in the Moonwort group) is a flattened, green, 
_ damp bricks. It is thin, and co of a single layer | 
= 
ee, 
Prothallus, or Prothallium—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 jin. 
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 
Fic. 292 ANTHEROZOID, much magnified. 
with a tuft of fine hairs, or cilia, at one end (see Fig. 
292). 
division goes on in the young antheridium, whereby — 
it is finally made up of a layer of cells surrounding 
Fic. 293. YOUNG ANTHERIDIUM, much magnified—c, Central Cell, 
—— — Parent Cells of Antherozoids; e, e, Epidermis of 
us. * 
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 
ing 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, 
e 
Fic. 294. RIPE ANTHERIDIUM (much magnified), from which 
Antherozoids have been shed by opening at o—ec, Empty 
Central Cell; e, 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. 
* 
| further divided into three layers; and these are again 
In most Prothalli, however, a process of sB- 
It increases in size, and becomes - 
