82 BOTANICAL GAZETTE [july 



Janeiro. He finds that, unlike the modern Marattiaceae, Psaronius has a thin 

 cortex bounded on the outside by a massive hypodermal sclerenchyma layer. 

 From the outer region of this sclerenchyma layer or from the epidermis strands 

 of tissue develop by a secondary activity of the cells, giving rise to a clothing 

 of multicellular hairs on the surface of the stem. Where the secondary roots 

 make their way through the cortex and sclerenchyma layer, they are limited, 

 as in modern Marattiaceae, by a definite epidermis and by a zone of disinte- 

 grated cortical cells. But after they have penetrated the sclerenchyma layer, 

 no such clearly marked boundary is perceptible, for here the roots pass down- 

 ward among the multicellular hairs on the outside of the stem. They are con- 

 sequently imbedded in the hairs which form a filling tissue between them, 

 closely applied to the stemward sides of the roots. Then, in turn, the hypoder- 

 mal layer of the cortex of the roots starts into activity. The resulting cells 

 are few on the inner surface of the roots, where the hairs from the stem are in 

 contact with them, while on the outwardly turned face they develop outgrowths 

 similar to the multicellular hairs of the stem. These in turn make an imbedding 

 layer for younger roots whose origin is higher in the stem, and which grow 

 downward over the root surfaces as the first roots grew over the stem. While 

 the hairs of the stem fill the crevices between the first roots and are soon over- 

 grown by them, similar outgrowths from the roots fill the spaces between the 

 successive layers of roots. Each system of hairs stops its growth in so short a 

 time that a meristematic part of the tissue can never be detected. No branch- 

 ing of the filling tissues appears, because of the constant correspondence between 

 the increase in the circumference of the stem and the number of cell rows in the 

 filling tissue, due to the increase in the number of points of origin. 



If it were possible to follow a root throughout its course, it would be found 

 to be organized in three parts: a proximal part, in which it breaks its way 

 through the cortex of the stem; a middle part, applied to the filling tissue 

 arising from the stem; and a distal part, in which the subepidermal cortical 

 tissue develops. The so-called "inner" and "outer" roots of Psaronius 



illustrate the two last mentioned portions. 



In an attempt to find whether this peculiar development of the outer cortex 

 is present in plants related to Psaronius, Solms-Laubach examined a stem of 

 Xylo psaronius. Though its poor state of preservation made definite con- 

 clusions impossible, the presence of the root of another plant between the 

 sclerenchyma layer of the stem and the inner roots is strong evidence of a 

 resemblance. In confirmation of this, Schuster 20 has shown complete cor- 

 respondence between a well-preserved root system of Xylopsaronius and 

 Psaronius. The tissue formerly interpreted as secondary xylem in the root of 

 Xylopsaronius is in reality secondary filling tissue originating from the cortex 

 like that described in Psaronius by Solms-Laubach. His photomicrograph of 



20 Schuster, J., Xylopsaronius, der erste Farn mit secundarem Holz ? Ber. 

 Deutsch. Bot. Gesells. 29:545-548. 1911. 



