THE ANGIOSPERMAE : ROOTS 829 



from the hypocotvl has lost even the form of a root and develops into a circular 

 crust on the rock, almost exactly like a Lichen. The shoots which arise 

 from these " roots " are endogenous, but they can scarcely be called 

 adventitious since they form part of the regular course of development. The 

 family presents some of the most interesting phenomena of adaptation to 

 environment in the whole plant world. 



The Evolution of Roots 



The possession of true roots is generally regarded as a distinguishing 

 character of vascular plants, but as a matter of fact some traces of root 

 formation can be found among the Bryophyta. A few species of Hepaticae 

 and Musci have leafless, branched, downward extensions of their axes which 

 it would be arbitrary to exclude from the category of roots since they corre- 

 spond both in their general morphology and their functions with those of 

 higher plants. There is indeed no reason why a gametophyte should not 

 develop roots like a sporophyte. xAmong the lower Pteridophyta, in which 

 the gametophyte and the young sporophyte are very much alike, it must 

 have been an open question at one period of evolution which would become 

 the rooted plant and the chief agent in the colonization of the land. 



In the majority of the Spermatophyta the embryonic plant normally 

 begins life with a main root which appears to be a downward prolongation 

 of the main axis, and in the embr>'OS of many Seed Plants the axis is a unitary 

 structure with growing points at both ends, while in the tissues between one 

 cannot distinguish the root and stem portions until a late stage of germination. 



This condition in the embryo has suggested that the primary root is 

 simply a downward extension of the primary axis. If we look, however, at 

 those members of the Thallophyta which normally grow attached to a sub- 

 stratum we see that the base of the axis terminates, not in a root, but in an 

 attaching organ or holdfast. In comparing these lowly plants with the 

 Spermatophvta we would gain the impression that the essential step in 

 evolution which has produced the root was the establishment of a basal 

 growing point, whereby the axis became bi-polar and acquired the power of 

 extending in both directions. But we would get a false idea of this evolution 

 if we were to imagine it as taking place in a fully evolved axis such as that 

 we are familiar with in the Seed Plants. The evolution of the root took place 

 among the lower Pteridophyta, and some of the lowest of that group, notably 

 the Psilotales, are rootless even at the present day. The evolution of the 

 root did not therefore coincide with the development of the land habit of 

 life but occurred among plants which were already land-living. Nevertheless 

 its importance is due to the conditions of land life, which restrict water 

 absorption to the underground parts. 



The lower end of the axis in the most primitive land plants — the " anti- 

 apex " as we may call it— did not form a root but a bulbous swelling, 

 corresponding to the thallophytic holdfast, which was well developed in the 

 fossil Psilophytales and perhaps survives among many of the Pteridophyta 



