ORIGIN OF THE ROOT-CAP 89 



same initial cell, and only become secondarily differentiated owing 

 bo the appearance of periclinal walls in the common mother-segments. 



In conclusion, attention must be specially directed to the various 

 complications that arise in connection with the origin of the root-cap, 

 in the case of roots which grow by means of several apical initials. 42 

 As has been stated above, the roots of the Marattiaceae grow by 

 means of four juxtaposed initials, and as regards the origin of the root- 

 cap resemble those of Equisefoim and most Filicales : the apical initials 

 in fact undertake the construction of the root- cap as a subsidiary 

 function, and to that end undergo occasional periclinal divisions. The 

 formation of a root-cap is not such a simple matter, where both vertically 

 and horizontally seriated initials occur together the normal condition 

 in the roots of Phanerogams. In order to obtain a clear understanding 

 of the great variety that obtains witli regard to the relations between 

 primordial meristem and root-cap, it is necessary to keep in mind the 

 circumstance that the root-cap is a secondarily evolved adaptive struc- 

 ture, which in the first instance constitutes a mechanically and generally 

 protective appendage of the root-tip, and in addition usually contains 

 the geotropic sense-organ of the root. The existence of a genetic 

 relation between the primordial meristem and the developing root-cap 

 must accordingly be regarded as a secondarily acquired feature. It is 

 impossible to give a coherent account of the variety of structure 

 exhibited by the growing-points of Phanerogamic roots, except from 

 this phylogenetic point of view. 



The various modes of origin of the root-cap in Phanerogams may 

 all be referred to one or other of the following six principal types of 

 development. 



Type 1. The root-cap arises from a special meristematic layer or 

 calyptrogen, which has no genetic connection with the main body of the 

 root. The apical meristem of the root proper thus appears sharply 

 delimited from the root-cap and calyptrogen ; if these could be lifted 

 bodily from the tip of the root, the surfaces of separation would be 

 quite smooth and continuous. This mutual independence of root-cap 

 and root proper is exemplified by the Gramineae, Cyperaceae (Fig. 

 IS), Juncaceae and Cannaceae. 



Type 2. The histogenetic layer of the root-cap is continued back- 

 wards into the protoderm (dermatogen) of the root proper ; or, in other 

 words, the single protodermal layer separates, as it approaches the apex, 

 at first into two and subsequently into three or more layers, of which 

 the innermost continues to add to the protoderm, while the others form 

 the successive shells of the root-cap. Eriksson has designated this type 

 of histogen a " dermoealyptrogen," because it is genetically related both 

 to the root-cap and to the protoderm of the root. From another point 



