92 MERISTEMATIC SYSTEM 



Type G. Here, as in the preceding type, the meristem s of root- 

 cap and root proper coalesce to form a common primordial nieristem. 

 The protoderm, however, behaves differently, inasmuch as it takes 

 lid part in the formation of the root-cap if, indeed, it is differentiated 

 at all. The sixth type thus bears the same relation to the fifth, as 

 the fourth does to the third. It is exemplified, according to Treub 

 and Flahault, by the Liliaceae, Aspidistreae, Ophiopogoneae, etc.: 

 examples which have been carefully studied are Oalla palustris, 

 Aiithi ricuiii ramosum and Allium. 



If the first of these six types of apical growth be ignored for 

 the moment, and the remaining five be considered in succession, a 

 sort of panoramic view may be obtained of the changes which the 

 root-tip of Phanerogams has undergone during the gradual evolution 

 of a cap or hood for the protection of the primordial meristem. In the 

 simplest case, the desired result is achieved by means of a proliferation 

 of the peripheral layer or protoderm (Type 2). A more serious 

 modification of structure is produced, where this hypertrophy extends 

 to the cortical layers (Types 3 and 4). The influence of the root-cap 

 thus tends to affect more and more deeply situated tissues of the 

 root. The organic and genetic connection between parent organ and 

 appendage is most intimate, where an initial zone common to the 

 two is developed (Types 5 and 6). 



There is a certain analogy, as will be shown later, between the 

 behaviour of the primordial meristem in different roots and the variations 

 in the mode of formation of cork. For cork, which is likewise a protec- 

 tive tissue, in certain cases originates in the epidermis, but in others arises 

 from hypodermal parenchyma or even from more deeply seated layers. 



Somewhat fuller consideration has still to be given to the first- 

 mentioned type of arrangement, which is characterised by the fact 

 that the root-cap is entirely independent of the main body of the root. 

 The answer to the question, as to how the root has come to possess 

 a root-cap at all in these circumstances, is provided by the phenomena 

 attendant upon the first differentiation of lateral roots. The early 

 history of the developing rootlets was thoroughly investigated long ago 

 from this point of view by Nageli and Lietgeb in the case of Oryza 

 sativa [Fig. 21a, b]. It appears that, in the first stage of development 

 of such a root, the root-cap originates from two cells of the endodermis 

 or innermost cortical layer of the parent organ : these initial cells, 

 which are steadily pushed outwards owing to the radial extension 

 of the developing root, divide at first by radial and later also by 

 tangential walls, and so give rise to the primary root-cap. The 

 structure subsequently becomes more complex, owing to the fact 

 that the terminal element of the median cell-row in the body 



