230 . Vegetable Tissue. 



Vascular tissue consists of simple membranous tubes, taper- 

 ing to each end, but often ending abruptly, either having a fibre 

 generated spirally in their inside, or having their walls marked 

 by dots or transverse bars arranged in a spiral direction. We 

 will describe the two principal kinds of vascular tissue, the Spi- 

 ral Vessels, and Ducts. The former are membranous tubes 

 with conical extremities, their inside being occupied by a fibre 

 twisting spirally, and capable of unrolling with elasticity. To 

 the eye, when at rest, they look like wire twisted round a cylin- 

 der that is afterwards removed. For the purpose of finding 

 them for examination, the stalk of a strawberry leaf, or a youn<* 

 shoot of Ihe Cornus Alba may be conveniently used ; in these 

 they may be readily detected by pulling the specimen asunder, 

 when they unroll, and appear to the naked eye like a fine cob- 

 web. Fig. 2, A spiral vessel ; fig. 3, the same magnified, show- 

 ing the elevated, and perhaps glandular border, the thread of 

 which is sometimes double or compounded of others. In size, 

 like all other kinds of tissue, they are variable, but generally very 

 small in the petals and filaments. An irritability of a curious 

 kind has been noticed in their fibres. Malphigi tells us that in 

 herbaceous plants and some trees, especially in the winter, a 

 beautiful sight may be observed by tearing gently asunder a 

 portion of a branch or stem still green, so as to separate the coils 

 of the spires, and the fibre will be found to have a downward 

 worm-like motion, which lasts for a considerable time. These 

 vessels are situated in that part of the axis of the stem surround- 

 ing the pith, and called the medullary sheath, and also in every 

 part of the tissue which originates from it, such as the veins of 

 leaves, and petals, and all other modifications of leaves. In the 

 stem of one class of plants, spiral vessels occur in the bundles of 

 woody tissue that lie among the cellular substance ; in the leaves 

 of some plants they are found in such abundance as to be col- 

 lected in handfuls in some islands of the West Indies, for spe- 

 cial purposes. We are also told that about a drachm and a 

 half is yielded by every plantain, and the fibres may be either 

 employed in the manufacture of a sort of down, or spun into 

 thread. Ducts are membranous tubes, with conical or rounded 

 extremities, their sides being marked with transverse lines or 

 rings or bars, or dots arranged spirally, and being incapable of 

 unrolling. In some states, these approach so nearly to the spiral 



