226 ABSORBING SYSTEM 



habit closely resemble genuine roots, are also covered with absorbing 

 hairs. Both in the Hymenophyllaceae and in Salvinia, however, 

 the root-hairs no longer represent mere prolongations of the super- 

 ficial cells, but are separated by cross-walls from the piliferous 

 elements. In Salvinia, in fact, each hair is divided into from seven 

 to nine compartments by a corresponding number of transverse 

 walls ; the terminal segment is short and conical, and ends in a 

 sharp point. 



It has already been stated that the absorbing tissue of the root 

 inclusive of the root-hairs is short-lived and deciduous. After its 

 disappearance the root is bounded on the outside by the outermost 

 cortical layer, which acquires suberised walls and thereafter constitutes 

 a continuous secondary epidermis, or, as it is usually termed, an 

 exodermis. If this suberisation takes place while the absorbing tissue 

 is still active (Coleus, Lamium, Heeler a ; most Monocotyledons), special 

 short cells with unsuberised walls and abundant protoplasmic contents 

 are interpolated at regular intervals between the ordinary long 

 exodermal cells ; the short elements serve as passage-cells for the 

 transmission of absorbed materials from the absorbing tissue to the 

 living parenchyma of the cortex. The most specialised types of 

 exodermis are found in aerial roots. 



B. RHIZOIDS. 121 



The possession of genuine roots is characteristic of the most 

 advanced stages in the differentiation of the plant-body. It is, in fact, 

 only the Phanerogams and the sporophytes of the Pteridophyta that 

 are furnished with true roots ; and even in these most highly organised 

 vegetable forms the root-system is not infrequently suppressed 

 in connection with some special adaptation. In addition to the 

 Thallophyta, which may be put on one side for the present, all the 

 Bryophyta Mosses as well as Liverworts and the gametophytes of 

 the Vascular Cryptogams are devoid of roots. Among the Bryophyta, 

 and in those Pteridophyte prothallia that are autotrophic, the functions 

 which among more highly organised plants are entrusted to the root- 

 system and its absorbing tissue, devolve entirely upon special trichomes 

 known as rhizoids. The simplest type of rhizoid is hardly distinguish- 

 able from a root-hair. The term " root-hair " is indeed often applied 

 to these characteristic gametophytic trichomes. There are, however, 

 grave objections to this extended definition of a root-hair. Physio- 

 logically considered, the trichomes in question represent both roots and 

 root-hairs ; that is to say, they are endowed with the properties of 

 typical root-hairs, but in addition possess many of the capacities of 

 roots ; in particular, rhizoids agree with roots in being sensitive to 



