184 



BOOKS AND CURRENT LITERATURE 



The circumstances which have determined the evolution of the her- 

 baceous type have received consideration (Eames, 1911). The chief 

 impetus in this evolution seems to have been supplied by the leaf trace, 

 a state of affairs corresponding well with the more familiar development 

 of the stele in Pteridophytes. In the Angiosperms adaptations to mod- 

 ern climatic conditions have been brought about by the formation of 

 storage parenchyma in the neighborhood of leaf traces. The plants 

 instanced by Eames in support of this view are the semi-herbaceous 

 Rosaceae, Potentilla, Sanguisorba and Geum, but the oaks also furnish 

 evidence pointing in the same direction. 



The older view concerning the ontogenj' of a wood}' stem seems to 

 have originated from a study of vines such as Aristolochia which show 

 a circular row of bundles in the young stem, but a more or less solid 

 stele after the lapse of a few years. In highly specialized plants like 

 vines the protoxylem groups are rather remote, but in Quercus there is 

 a fairly continuous woody cylinder from the first. 



The influence of the leaf trace has been felt also in plants which in 

 the course of evolution have retained the arboreous habit. In the oaks 

 the accumulations of storage parenchyma above and especially below 

 the point of entrance of a leaf trace have given rise to the broad rays 

 which give oak wood its characteristic grain. As stated earlier, the 

 seedling oak possesses only uniseriate rays (Eames, 1910); these gradu- 

 ally approach each other in groups, and fusion finally occurs, with 

 the production of what may properly be called a compound ray. Species 

 of Alnus show aggregations of narrow rays, the so-called "false rays," 

 which represent stages of incomplete fusion of narrow rays. It is accord- 

 ingly improper to speak of the broad rays of Quercus as "primary," 

 for they represent fusions, not the original rays of the young stem. 

 Groom (Ann. Bot. 25. October 1911) has raised certain objections to 

 this theory of origin of broad rays, and his arguments have been con- 

 sidered by Bailey in his paper of 1912. 



Further stages in what may be called the evolution of wood are shown 

 in Fagus and many other genera in which fusiform or multiseriate rays 

 occur. Thompson has shown, ^ that in Casuarinaceae, Fagaceae and 

 other families a dissection of compound rays has occurred, resulting in 

 the production of multiseriate rays. Stages in this process are found 

 in seedlings, roots and other parts. 



* Thompson, W. P., On the origin of the multiseriate raj- of the Dicotyledons. 

 Ann. Bot. 25: 1005-1014, pis. 77-78, October 1911. 



