DIV. i PTEEIDOPHYTA 527 



The spores of Lycopodium davatum and other species are sometimes 

 used in pharmacy. 



Order 2. Psilotaceae 



The only representatives of this order are Psilotum (two tropical species) and 

 Tmesipteris (one Australian species). They show in some features relationship 

 with the Sphenophyllinae, but are most naturally placed with the Lycopodinae. 

 The complete absence of roots is noteworthy. The simple leaves are alternately 

 arranged on the dichotomous stems ; the fertile leaves near the tips of the branches 

 are always deeply divided and resemble a pair of leaves. 



Order 3. Selaginellaceae ( 128 ) 



To this order belongs the genus Selaginella, represented by numerous and 

 for the most part^ tropical species. They have, as a rule, profusely-forked, 

 creeping, and sympodially- branched stems, but occasionally erect branched 

 stems ; some form moss-like beds of vegetation ; others, climbing on adjacent 

 plants, possess stems several metres long. Certain xerophilous species (S. 

 lepidophylla in tropical America, etc.) can endure drying up for months or 

 even years, closing together their rosette - shaped shoots by a cohesion- 

 mechanism, expand again on the arrival of rain ( 127 ). In general the 

 Selaginellas are similar in habit to the Lycopodiums. They have small 

 scale-like leaves which usually exhibit a dorsiventral arrangement, such as is 

 shown, for example, in the alpine Selaginella helvetica (Fig. 494),. the stem of 

 which bears two rows of small dorsal or upper leaves, and opposite to them 

 two rows of larger, ventral, or under leaves. (Cf. also Fig. 134.) The 

 rhizophores ( 128 ) are organs that are peculiar to the plants of this order ; they 

 are cylindrical, leafless, shoot-like structures, which arise exogenously, usually 

 in pairs, from the stem at a bifurcation. At their ends a number of endogenous 

 roots are produced, but the rhizophores are able, when the normal shoots are 

 cut back, to continue their growth as shoots of ordinary construction. Even 

 below the first leaves of the seedling plant short rhizophores are formed, 

 from which the first roots arise endogenously. The leaves of Selaginella are 

 characterised by the presence at their base on the upper side of a small 

 membranous ligule. This serves as an organ for the rapid absorption of 

 water (rain-drops) by the leafy shoot ( 129 ). In many species of Selaginella the 

 epidermal assimilatory cells of the leaves possess, as in Anthoceros, only one large 

 chloroplast ( 13 ). 



The cones or flowers are terminal, simple or branched, radially symmetrical, 

 or less commonly dorsiventral. Each sporophyll subtends only one sporangium, 

 which springs from the stem above the leaf-axil. The same spike bears both 

 macrosporangia and microsporaugia. Each macrosporangium (Fig. 495 A -C) 

 contains only four macrospores, which are produced by the growth and division 

 of a single spore -mother-cell ; all the other mother-cells originally developed 

 ultimately disappear. On account of the increasing size of the spores the 

 spherical macrosporangia become nodular. Opening, which is due to a cohesion- 

 mechanism, occurs along definite lines of dehiscence, the wall splitting into two 

 valves, which curve back from a boat-shaped basal portion. The spores are 

 ejected by the pressure of the contracting boat-shaped part and the valves. 

 Numerous spores are formed in the flattened microsporangia. The mode of 

 dehiscence is similar in these also, but the boat-shaped portion of the wall is 

 smaller, the valves extending to the base. 



