Though it is impossible in many cases to form an estimate of the age of individual plants, there are clear indications that some specimens afford notable instances of longevity. Chamberlain estimates the age of some plants of _Dioon spinulosum_ as exceeding 400 years and mentions an example of _D. edule_ that is probably 1000 years old. An unusually tall plant of _Encephalartos_ in the Botanic Garden of Amsterdam is believed by Prof. de Vries to have reached the venerable age of 2000 years[36]. The restricted range and in many cases the solitary existence of recent Cycads, with their tall stems clothed with the persistent cork-covered stumps of thousands of fronds, deepens the impression of antiquity derived from a study of the geological history of this dwindling race. _Stems._ The tall columnar stems of some species of _Cycas_, often branched or bearing numerous ovoid buds like enlarged bulbils[37], are characterised by the regular alternation of large and small leaf-bases as seen in the stem of _C. circinalis_ reproduced in fig. 380. In older stems of this species the leaf-bases are exfoliated and the stem is covered with wrinkled and fissured cork; but in _Cycas revoluta_ the leaf-bases are even more persistent. The columnar but relatively stout stems of _Encephalartos_ (figs. 379, 382, 386, A) and _Ceratozamia_ are similarly encased in a covering of petiole-bases, but in these genera the differences between foliage-leaves and bud-scales is much less obvious and there is no zonal alternation. On the stems of _Macrozamia_ the rhomboidal leaf-bases are more uniform in size and there are no scale-leaves. The tall and often palm-like stems of _Microcycas_ sometimes show transverse rings on the bark marking the position of former terminal buds, and in older trunks these may disappear, leaving a fissured bark[38]. In _Cycas siamensis_ the tuberous stem is similarly covered with a rough bark (fig. 383) and the stems of _Zamia_ are also characterised by an absence of persistent leaf-bases (figs. 381, B; 395, I_a_, _a_). It is pertinent to remind the palaeobotanical student of the occurrence of flowering plants with stems closely simulating those of some Cycads. Prof. Bower[39] in describing _Rhynchopetalum montanum_, an Abyssinian Lobeliaceous plant, drew attention to the similarity in surface-features and to some extent in anatomical structure to cycadean stems. The resemblances are further emphasised in a more recently published account of the same species under a different name, _Lobelia Rhynchopetalum_[40]. [Illustration: Fig. 382. _Encephalartos Ghellinckii._ (¹⁄₁₁ nat. size.)] [Illustration: Fig. 383. _Cycas siamensis._ (From the _Encyclopaedia Britannica_.)]