The axis of the tubercles of Coryphantha spp. always points to the centre and to the base of the sprout. For the description of this position of the tubercles, the term "porrect" is used (see Fig. 7).
Within this strict orientation of the tubercle axis,three variations can be differentiated:
• If a part of the plant body is below ground level, the tubercles appear more upright and are described by this term (see Fig. 8).
• In certain species only the base of the tubercle points to the centre of the body, the upper part is more appressed and the direction of the tubercle axis on the whole is tangential to the surface of the plant body. Therefore, for this type, the term "appressed" is used (see Fig. 9).
• In clavate and columnar plants this basal central point becomes more and more elongated and becomes a line (see Fig. 10).
Fig. 10. Direction of tubercle axis, tubercles in clavate and columnar forms. Example: C. octacantha
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