2^2 



LECTURE XIV. 



organs serve as conducting organs for the transpiration-current, though they are at 

 the same time of use as elastic and rigid masses. 



The wood and lignified sclerenchyma are certainly not the only tissues which 

 can conduct water within the plant; for simple reflection shows that large fruits, 

 as those of the Gourd, absorb enormous masses of water into their parenchymatous 

 tissues, though no corresponding lignified strands pass through them from the 

 stem. The absorption of water in all young buds, as well as in soft non-lignified 

 napiform roots and tubers, shows that a movement of water is likewise possible, in 



Fig. 195. — Transverse section of a vascular bundle in the stem of Zea Mays. It consists of tlie parts 

 v,ff,s r, L ^^ thin-walled parenchyma of tile fundamental tissue Apart from the lignified cells between 

 g- axiAff, only the dark shaded sclerenchymatous sheath which surrounds the vascular bundle constitutes a 

 woody layer worth mentioning and apparently suitable for the conveyance of the ascending current of water. 



parenchymatous tissue, even without the presence of lignified cell walls. The dis- 

 tinguishing feature of the wood, however, is the great rapidity with which the 

 particles of water can move in it; and just this rapidity is necessary where the 

 transpiration -current of land-plants here considered is concerned. Hence we see that 

 even very highly organised plants, when they do not transpire, or do so but little, 

 possess no woody bundles, or only very thin ones — as, for instance, submerged and 

 floating phanerogamous water plants, and those root parasites which develop chiefly 

 under ground, as the Balanophoreae, &c. The lignified layers and strands in the 



