l83 SIEVE-TUBES. 



sieve-tubes. Neither Hegelmaier ^ nor I could find the clearly-latticed sieve-plates, 

 almost like those of Pteris aquilina, which Dippel described on their lateral walls. 

 On the other hand I saw on the whole lateral wall numerous small pits, solitary or in 

 groups, to which were attached those peripheral granules, like those of Pteris 

 aquilina, which turn yellow with iodine : but from these it could not be determined 

 whether there are open sieve-pores or not (comp. also Chap. VIII). In the smaller 

 Lycopodia, the Selaginellae, and in very many Ferns with small vascular bundles 

 composed of narrow elements (comp. e. g. below, Fig. i6o, Polypodium vulgare), 

 the position in which the sieve-tubes occur in the above instances is occupied by 

 elements of similar form, and general character of contents and walls, but without 

 distinct sieve-plates or sieve-pores. Whether the latter are really absent, and whether 

 these elements are only the morphological equivalents of sieve-tubes, remains to be 

 further investigated both in these cases and also in Lycopodium. I am the less 

 inclined, in the doubtful cases, to deny the presence of sieve-tubes, and prefer the 

 more to treat the question as an open one, because these very organs counsel one 

 to be circumspect, for twenty-two years ago no botanist, with the exception of Hartig, 

 had any idea of the characteristic structure of the most conspicuous of them. 



■ Botan. Zeitg. 1872, p. 778. 



