>9°9l 



STOKEY— ANATOMY OF ISOETES 33* 



mon with modern lycopods. The spore -producing members, m 

 structure and development, are unquestionably of the lycopod type, 

 and as such present no obstacle to the retention of the group in the 

 Lvcopodiales. With reference to the anatomy my work would seem 

 to strengthen the position of Isoetes in the Lycopodiales. The irregu- 

 larities of its structure are not of such a nature as to isolate the group. 

 A>ide from the possible lack of a cauline portion in the stele, the 

 irregularities of its anatomy are limited to the absence of primary 

 phloem in the stem ; the absence of secondary phloem ; the lack of 

 differentiation into protoxylem and metaxylem in the stem; the 

 large amount of parenchyma in the secondary wood ; and the imperfect 



development of the wood. 



The absence of primary phloem seems to be characteristic of Isoetes. 

 There are no indications of it in the four species described in this 



paper 



Hystrix 



to identify primary phloem with certainty." Although the absence ot 

 primary phloem is recorded for juvenile pteridophytes (e.g.Matonia 

 pectinata) by Tansley and Lulham, (23, p. 482), so far as is known 

 it is present in the stem of all other adult pteridophytes. In this 

 Point, then, Isoetes stands alone. The presence of phloem in the leat 

 traces and roots, and the collateral arrangement of the bundle in 

 the lower part of the leaf trace indicates a descent from a line in 

 which the phloem is present in the stem as a layer around the xylem. 

 ^ is difficult to tell what importance to attach to the tendency ot tne 

 'eaf trace to become concentric in the middle and upper part; and 



fn regard to the next point, the absence of secondary phloem 1 

 ** with a cambium, the isolation of Isoetes is less certain. * 

 2 °, P- 167) in discussing the secondary growth of the I*P ldoden T^ 

 *JS : " Although the presence of primary phloem can be «****. 

 *ith certainty, some doubts have been expressed as to the produc 10 

 of >econdary phloem by the cambium." It may be well to *&*"* 

 t00 » that there are cases among the modern pteridophytes in wineo 





^condary xylem is produced, but no secondary p nloem ^ 

 Served. This suggests that when reduction occurs in a form 

 ec °ndary thickening, the disappearance of the phloem precede 

 of the xylem. 



