56 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



practise the mode of nutrition of normal vascular types, namely, 

 photosynthesis together with absorption of the necessary elements 

 (usually in simple compounds) from the air and soil, or, when the 

 plants are aquatic, in solution with the water in which they live. 

 In common with almost all of the plants remaining to be described, 

 the main food-storage substance is starch. 



These relatively large plants are the sporophytes, which comprise 

 the main, dominant generation in this and all the remaining groups. 

 The spores are produced in special organs (sporangia) developed on 

 short outgrowths which are compacted into distinct, cone-like 

 ' strobili ' that are commonly developed at the tops of the stems. 

 The spores are all alike but unique in having attached to them at 

 one point four slender processes that bend or straighten rapidly with 

 changes in atmospheric humidity, consequently often causing the 

 spores to move. On germination the spores produce a gametophytic 

 body called a prothallus. This is small and possessed of rhizoids, 

 irregularly branched, green and photosynthetic, and usually each 

 one produces organs of only a single sex. After fertilization by the 

 peculiar spiral, multiflagellate swimming spermatozoid, the egg 

 develops in situ into a young sporophyte plant which soon becomes 

 independent, so completing the life-cycle. 



In spite of the limited number and variational range of living 

 forms. Horsetails occupy a considerable array of land and freshwater 

 habitats and are geographically very widespread, extending from the 

 tropics to the highest latitudes of land. They especially favour 

 marshes, lakesides, and damp sand or silt, which they may colonize 

 aggressively ; but as components of more mature vegetation they 

 are very minor. The outer part of the stem is often heavily 

 impregnated with silica, which has led to some species being widely 

 used for scouring pots and pans — hence their alternative name of 

 Scouring-rushes. 



Lycopodineae : These include the living Club-mosses (Ground- 

 pines), Spike-mosses, and Quillworts, as well as numerous huge 

 trees of earlier geological ages that are now known only as fossils. 

 The present-day representatives are lowly and herbaceous, differ- 

 entiated into stem, roots, and leaves, the last being numerous and 

 usually small as well as simple. The stems are rarely more than a 

 matter of inches in height or feet in length. Most species are 

 evergreen, overwintering without the aerial parts dying back. The 

 stems are trailing or upright and usually branched, or, in the Quill- 



