PAST HISTORIES OF PLANT FAMILIES 97 



indicates with considerable exactness the position of 

 any plant, so that, as these are known, we can deduce 

 from them, with a feeling of safety, the position that 

 Poroxylon takes in the natural system. In its anatomy 

 the characters are those of the Cordaiteae, with certain 

 details which show a more primitive nature and seem 

 to be characteristic of the groups below it in organization. 



Poroxylon is not common, and until recently had not 

 been found in the Lower Coal Measures of England. 

 The plants appear to have been much smaller than 

 Cordaites, with delicate 

 stems which bore relatively 

 large simple leaves. The 

 anatomy of the root was 

 that common in Gymno- 

 sperms, but the stem had 

 a very large pith, and the 

 leaves were much like those 

 of Cordaites in having 

 parallel veins. An im- 

 portant character in the 

 anatomy of the stem was 

 the presence of what is 

 known as centripetal wood. 

 This must be shortly explained. In all the stems hitherto 

 considered, the first -formed wood cells (protoxylems, 

 see p. 57) developed at the central point of the wood, 

 towards the pith (see fig. ig, px, p. 49). This is char- 

 acteristic of all Angiosperms and the higher Gymno- 

 sperms (except in a couple of recently investigated 

 Pines), but among the lower plants we find that part of 

 the later wood develops to the inner side of these pro- 

 toxylem masses. The distinction is shown in fig. 65. 



This point is one to which botanists have given 

 much attention, and on which they have laid much weight 

 in considering the affinities of the lower Gymnosperms 

 and the intermediate groups between them and the ferns, 

 which are found among the fossils. In Cordaites this 



(0 122) 3 



Fig. 65. — A, Normal bundle uf higher 

 plant ; x, protoxylem on inner side next the 

 pith p, and the older wood w outside it, cen- 

 trifugal wood. B, Bundle with wood cells c 

 developed on inner side of protoxylem, cen- 

 tripetal wood ; the arrow indicates the direc- 

 tion of the centre of the stem. 



