PAST HISTORIES OF PLANT FAMILIES 



143 



content to consider them as a form of root-bearing stem, 

 practically confined to the Lycopods and principally 

 developed among the palaeozoic fossils of that group. 



In microscopic structure the rootlets are extremely 

 w_ell known, because in their growth they have pene- 

 trated the masses of the tissues of other plants which 

 were being petrified and have become petrified with 

 them. The mass of decaying vegetable tissue on which 

 the living plants of the period flourished were every- 

 where pierced by these intrusive rootlets, and they are 

 found petrified inside other- 

 wise perfect seeds, in the 

 hearts of woody stems, in 

 leaves and sporangia, and 

 sometimes even inside each 

 other! Fig. 95 shows such 

 a root r lying in the space 

 left by the decay of the soft 

 tissue of the inner cortex in 

 an otherwise excellently pre- 

 served Lepidodendron stem 

 (see also fig. loi). In fig. 

 loi their simple structure 

 is seen. They are often ex- 

 tremely irregular in shape, 



owing to the way they seem to have twisted and flat- 

 tened themselves in order to fit into the tissues they 

 were penetrating. No root hairs seem to have been 

 developed in these rootlets, but otherwise their structure 

 is that of a typical simple root, and very like the swamp- 

 penetrating rootlets of the living Isoetes. 



The Stigmarian axes and their rootlets are very 

 commonly found in the "underclays" and "gannister" 

 beds which lie below the coal seams (see p. 25), and 

 they may sometimes be seen attached to a bit of the 

 trunk growing upwards through the layers. They and 

 the aerial stems of Lepidodendron are perhaps the 

 commonest and most widely known of fossil plants. 



Fig. loi. — Transverse Section through 

 a Rootlet of Stigmaria 



oc. Outer cortex; i, space; ic, inner 

 cortex; w, wood of vascular strand (wood 

 only preserved) ; px, protoxylem group. 



