68 FOSSIL PLANTS. 



tropical or subtropical countries, the boreal regions must have enjoyed a similar climate 

 when they were inhabited by these ancient elephants. It was, however, discovered that 

 their skin was clothed with wool and long hair, and that consequently they were adapted 

 to endure a cold climate. In plants the structure of the fruit would, in most cases, teach 

 nothing as to the temperature and humidity of the atmosphere in which, or the kind of 

 soil upon which, the organism grew, though it would be of the first importance in deter- 

 mining the systematic position. On the other hand, the root, the leaves, and the tissues 

 of the plant would be of only secondary importance in regard to systematic position, but 

 of the highest value in determining physical condition. In regard to Lepidodendron, its 

 singular roots would seem to imply that it derived a large amount of moisture through 

 them from a moist soil, and so far differed from most living Cryptogamia which obtain it 

 mostly from the atmosphere. The roots of this genus presented in their crowded and 

 long rootlets an immense surface for the absorption of moisture ; and, in their great 

 abundance of lax cellular tissue, possessed the means of containing this moisture and 

 transmitting it to the foliage. 



" The leaves of Lepidodendron were simple, lanceolate, acute, and sessile. They had 

 a single medial nerve. The younger branches were densely crowded with leaves ; and the 

 scars left on the trunk, after they perished, give the numerous beautiful markings by which 

 the species have been distinguished. The leaves, when found separated from the branches, 



are called Lepidophylla 



. " The structure of the arboreal stem of Lepidodendron is much more complex 

 than that of any known cryptogam. The central axis of the irregularly arranged vascular 

 tissue in Lycopodium is suited to the low stature of the plants of that genus ; but in the 

 giant Lepidodendron there is a complexity which approaches the structure of some dico- 

 tyledonous stems. The general arrangement of the tissues, resembling what exists in some 

 Cycadea and Cactacea, has caused this fossil plant to be referred sometimes to the one 

 and sometimes to the other of these Orders ; but the resemblance is only one of analogy 

 and not of affinity. The presence of scalariform tissue, of which the woody portion is 

 entirely composed, and the absence of medullary rays, would, even if the fruit were 

 unknown, be sufficient to establish the cryptogamic nature of the plant. A comparison 

 with the Cycadean stem may help us, by the resemblances and differences which will 

 appear, better to understand the stem of Lepidodendron. The Cycads have all a large 

 medulla, composed of large-sized parenchyma, in some genera traversed by numerous 

 vascular bundles, as in Encephalartus, and in others entirely cellular, as in Cycas and 

 Zamia. This is surrounded by a single woody cylinder, or several, everywhere penetrated 

 Avith medullary rays. Beyond this there is a considerable thickness of parenchyma, com- 

 posed in their inner portion of cells whose length exceeds only slightly their breadth ; 

 these gradually lengthen, until they assume an appearance very like the external portion 

 of Lepidodendron. This cortical parenchyma is traversed by the vascular bundles which 

 supply the leaves. The two stems are evidently built upon the same plan ; and were we 



