PSILOTACEAE 
548 
sporangia are 2- or 3-loc., borne on small two-lobed sporopliylls. 
“There lias been much disagreement as to the morphological nature 
of the sporangiophores of the Psilotaceae. The two chief views are 
the following: (1) that the whole sporangiophore is a single foliar 
member ; (2) that it is a reduced axis bearing a terminal synangium 
and two leaves. The recent very careful researches of Bower upon 
the origin of the sporangiophore and synangium confirm the former 
view” (Campbell; see also Bower in Phil, Trans . clxxxv, 1894, 
P- 473)- 
“The fully-developed synangium (2-loc. in Tmesipteris, 3-loc. in 
Psilotum) has the outer walls of the loculi composed of a superficial 
layer of large cells, beneath which are several layers of smaller ones. 
The cells composing the septa are narrow tabular ones ; occasionally 
the septum is partially absent.... Bower regards the whole synangium 
as homologous with the single sporangium of Lycopodium ” (Camp- 
bell). The prothallus has not been seen. 
Psilotum Sw. Psilotaceae. 2 sp. trop. They are probably saprophytic 
in their habit, and have neither roots nor green leaves, but only green 
stems. See order for details of sporangia &c. Vegetative reproduction 
is common, small gemmae being formed upon the rhizomes. At first 
no structural differentiation is visible in these, but apical cells are 
formed later. 
Psoralea Linn. Leguminosae (in. 6 ). 100 sp. Afr., As., Austr. 
P. esculenta Pursh (N. Am.) is the prairie turnip, with an edible 
tuberous root. 
Psychotria Linn. (excl. Mapourea Aubl.). Rubiaceae (11. 15). 360 sp. 
trop. Some are heterostyled. Many have the infl.-axis brightly 
coloured. See Cephaelis. [For P. Ipecacuanha Stokes see Ura- 
goga-] 
Ptelea Linn. Rutaceae (ix). 7 sp. N. Am. P. trifoliata L. is often 
grown in parks (shrubby trefoil). Firs* monoecious. Fruit winged 
(cf. Ulmus). 
Pteridophyta. Vascular Cryptogams, one of the four chief divisions 
of the Vegetable Kingdom. They have a well marked alternation of 
generations, the oophyte (gametophyte or sexual generation) being 
insignificant in size compared to the sporophyte (asexual generation), 
but still capable of independent growth. The life history of a typical 
Pteridophyte may be shown diagrammatically as follows, taking the 
fern as an example: 
Fern-plant -»■ sporophylls sporangia spores 
t \ 
fertilised ovum — 5 spermatozoid - antheridia j othallus . 
( ovum archegoma ) 17 
The ‘plant’ or asexual generation alternates with the prothallus or 
sexual generation. In many P. there are male and female prothalli. 
The prothallus corresponds to the ‘ plant ’ in a moss or liverwort, 
