ORGANIC EVOLUTION 



143 



Fig. 89. Chickweed. 



beginning with the sporophyte phase — the leafy plant, 

 which, as with the preceding group, is the phase we ordi- 

 narily see. 



Any familiar herb, like the chickweed 

 (fig. 89), will show how much the 

 sporophytes of the two groups have in 

 common. The ordinary differentiation 

 of the plant body into root stem and leaf 

 is already very familiar. The plant 

 body is covered over with epidermis, 

 some of whose cells develop in the air 

 into plant hairs and in the soil into rhiz- 

 oids. If we strip a bit of epidermis from 

 a leaf we find its constituent cells, and 

 the guard cells of the breathing pores to 

 be of the same type as in the fern leaf. 

 If we section the leaf (fig. 90), we find 

 the same tissues in the same 

 relations. There are stoma- 

 tes in both layers of epi- 

 dermis and there are inter- 

 communicating air spaces 

 throughout the interior of 

 the leaf and here and there 

 are minute vascular bundles 

 in the midst of the paren- 

 chyma. 



In a cross section of the 

 stem there appear some 

 differences of importance. 

 Epidermis covers it (fig. 91a) and parenchyma fills most 

 of the interior as before, but the arrangement of the 

 vascular bundles is very different. There is hardly any 

 development of supporting tissue outside of the vascular 



Fig. 90. Cross-section of a bit of chick- 

 weed leaf, p, p. p, pores; e, epidermis; 

 X, crystal (probably of oxalate of lime). 



