]Sa 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 Selaginella;, 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. 



