﻿PRESERVATION AND EXTERNAL CHARACTERS. 35 



It may be casually noted that the Cycadese owe a portion of their greater 

 height, in the case of the female plants, to the fact that in fructification, as one may 

 say, the summit grows forward, continuing the trunk structure as an immense 

 strobilus. The carpellary leaf bases also being persistent and alternating with the 

 scale leaves and the foliage leaf bases, then form a third armor-making series. In 

 the Zamiese, on the contrary, only the relatively few peduncle bases are left behind 

 to add to the alternating series of scale and foliage leaf bases forming the armor. 

 Contrariwise, it is not probable that a trunk of the Cycadeoidean type would ever 

 reach any great height ; for the heavy growth of ramentum about the leaf bases, 

 together with the disturbance of the armor due to the emergence of the lateral 

 fructifications which were certainly present in C. Jenneyana, the tallest known 

 member of the group, would not favor a tall habit. 



Plainly the limits of size in plants with a large pith, a weak woody column, 

 and a cortex and armor occupying more than one-half of the area of the transverse 

 section of the trunk must be distinct. Any failure of the leaf bases, peduncles, or 

 fruits, or the copious ramental hairs borne by and enveloping these organs, to 

 fairly occupy all the space exterior to the cortex would not only give opportunity 

 for decay, but weaken the trunk itself; and in accidents or arrests of growth, or 

 destruction of immature fruits, due to the attack of animals to which these plants 

 may have been subjected, there is suggested a very direct cause of such failure. 

 Aside from any physiologic reasons, ordinary mechanical factors are therefore sug- 

 gested in strictest accord with the conclusion reached in a later chapter that the 

 production of numerous lateral fructifications was the culminant and closing event 

 in the life of many of the trunks. Again, the roots of these plants were doubtless 

 diffuse or filamentous, a fact likewise more or less unfavorable to their reaching 

 great size. Hence it is interesting to find that some of the more distinctly columnar 

 trunks compensated in part for the weakness of the armorial zone by an immense 

 development of xylem. The lower interior portion of a trunk secured at Black 

 Hawk, 20 cm. in diameter and almost denuded of armor and cortex, looks mark- 

 edly like the stump of some hardwood tree-trunk with an unusually large pith. 

 (See upper photograph of plate xiv.) At the basal end the medulla is 5 cm. in 

 diameter and the xylem zone fully 10 cm. in thickness. At the summit of the 

 fragment, 30 cm. above the basal end, the medulla is not quite 6 cm. across, and 

 the xylem about 8 cm. in thickness. This does not, however, necessarily mean 

 tapering of the complete trunk, but rather that the cortex and armor of large leaf 

 bases formed after the trunk attained its full diameter are relatively thicker, the 

 actual diameter of the trunk remaining quite constant, as seen in the various bases 

 and segments of trunks mentioned above, thus fulfilling the requirements of strength. 

 That is to say, with increase in height there is a compensating strengthening of the 

 xylem at the base, just as in an ordinary tree-trunk. 



It is only necessary to add that the limitations in size among the cycads are 

 very much the same as are seen in the tree ferns. The short and robust Angiop- 

 teris evecta (cf. fig. 5), with trunks approaching a meter in diameter and a meter 

 more or less in height, compare very closely in size with such trunks as Cycadeoidea 



