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A TEXTBOOK OF THEORETICAL BOTANY 



I 



definite calyptrogen is formed in a few Monocotyledon families, such as the 

 Gramineae, in which it is completely distinct from the other meristematic 

 zones, so that the root cap is really an independent structure (see below). 

 In the majority of Dicotyledons the calyptrogen is formed by the division of 

 the protoderm cells or from the protoderm and periblem together, so that the 

 root cap must be regarded as an outgrowth of these tissues. Among the 

 Leguminosae and the Liliaceae, however, there are a number of genera in 

 which no distinct calyptrogen exists, and the root cap arises from the same 

 initial group as the other root tissues, with which, consequently, it is more or 

 less directly continuous. Even in these cases the protoderm often contributes 

 to build up the root cap tissues. From the point of view of evolution it seems 



probable that the root cap is a development from the 

 protoderm and that the normal dicotyledonous condition, 

 described above, is the most primitive. 



Before leaving the origin of the root cap, mention 

 should be made of the condition in lateral roots and in 

 adventitious roots which develop endogenously. Here 

 a protective pouch is formed over the apex of the 

 young root by the endodermis of the parent axis, which 

 becomes meristematic at this point. Normally this pouch 

 is later sloughed off, but in many Monocotyledons, e.g., 

 the Gramineae, no true calyptra is formed by the root 

 itself and the pouch remains as a permanent structure, 

 growing from its own inner layer, which constitutes the 

 calyptrogen. The remarkably large root " cap " in 

 Lemna is of this type (Fig. 792). 



The cells of the root cap are cut off from the meri- 

 stem in radial rows, but they very quickly become vacuo- 

 lated and rounded off, the walls at the same time 

 becoming mucilaginous. They are so loosely attached 

 to each other that they are easily rubbed off by contact 

 with the soil. Thus the life of each cell is short and the tissue is con- 

 tinuously renewed. 



A small central core or columella of cells in the root cap contains starch 

 grains, to which the special name of statoliths has been given, as they have 

 been regarded as the means by which the root orientates itself to gravity. 

 The cells of the root apex are much less permeable to water than those of 

 the stem apex, and it may be due to this that the initial group is internal and 

 nearer to the apex of the vascular system than in the stem. The same fact 

 may also explain the rapid maturity and short life of the external tissues of 

 the cap. 



The protective function of the root cap is illustrated by the remarkable 

 power of penetration shown by roots, not only in soil and rock crevices, but 

 also in boring through other plant tissues. The adventitious roots of some 

 plants bore their way downwards through the cortex of the stem to the 

 ground level. Tissues of neighbouring plants are also sometimes penetrated, 



Fig. 792. — Lemna 

 minor. Apex of root 

 showing abnormally 

 large root cap. 



