4i6 BOTANICAL GAZETTE [december 



trunk, like an epidermis [jig. 4, c^). After the root is about one- 

 fifth grown the inner layer develops no further and is torn away 

 at the base, to be carried downward as a cap in the further growth 

 of the root, the upper part of the trunk therefore being left 

 naked. 



It may be worthy of remark that in its younger stage the root 

 of Azolla filiculoides thus presents a state of things analogous to 

 that in typical dicotyledons, since the superficial layer of the 

 body of the root is derived from the calyptrogen — if we may so 

 speak of the original cap segment (ci); whereas the mature root 

 is like that of other vascular crvptogams and monocotyledons 

 among flowering plants in exposing an unprotected cortex back 

 of the root-cap. 



In all cases examined by me there is an extra periclinal divis- * 



ion at the apex of the inner cap {^fig^ 4., d). 



In Azolla caroliniana I find that sheath and cap behave as 

 stated iox A . filiculoide s , except that the second periclinal division 

 (d) is absent. 



THE ROOT-HAIRS. 



In the majority of vascular plants root-hairs are formed by 

 the external cells of the root in the region which is ceasing or 

 has ceased to grow, where cell-division has been suspended and 

 the tissues have become fixed. Here any or all of the cells 

 without distinction of form, size, or contents may send out the 

 organs of absorption. Only one exception was known to 

 DeBary, that of Lycopodium, where the hair initials are set off 

 in the still plastic epiblema not far behind the apex, and these, 

 remaining short while the intervening cells elongate, alone pro- 

 duce tubular outgrowths. For the sake of comparison a figure 

 of these hair initials and of the resulting structure in Lycopo- 

 diiim lucidulum is given {fig. 11), Dc Bary overlooked Bruch- 

 mann's observations'' on the production of root-hairs in essen- 

 tially the same manner in Isoetes, by the early establishment of 

 special cells for the purpose. 



The initials of the trichomes in the root of Azolla filiculoides 



^Bruchmann, H., Ueber Anlage und VVachsthum der Wurzein von Lycopodium 

 und Isoetes. Jenaische Zeitsch. Natxirwiss. 522. 1874. 



( 



