THE TISSUES OF THE STEM 4; 



was derived. That septum was in fact occupied by .1 large round 



pit, of which the margin remains as the ring, while the thin pit- 

 membrane has been absorbed. Further inwards the vessels are 

 successively smaller, and show very characteristic markings. In this 

 particular case there is first another pitted vessel, but narrower, 

 with the pits arranged with some spiral indication. In the inner 

 vessels complete spiral bands of thickening are coiled within the 

 thin wall. In the successive vessels nearer the pith the spirals are 

 more loosely coiled, till finally there is no continuous spiral, but a 

 series of more or less regular rings (annular vessels). Such vessels 

 are characteristic of protoxylem, and they abut directly on the paren- 

 chymatous pith. 



The structure of the protoxylem is a consequence of the earliest vessels 

 being developed while the stem is still growing in length. While the thickening 

 is being deposited upon their walls they are being constantly stretched, and 

 those earliest formed will be stretched most. The annular thickening appears 

 in these. The closer spirals of those formed later show that they have been 

 stretched less. Finally, where the stretching has been slight, the spirals run 

 together laterally, leaving only irregular pits between them. The thickening 

 rings and spirals are effective in keeping the cavity of the vessels open against 

 the pressure of the surrounding cells. On the other hand, a sufficient area 

 of thin pit-membrane is necessary to allow of the exit of water from the 

 vessels to them. 



Stems of Aquatic and Climbing Plants. 



The plan of construction of the vascular bundle of Scroplmlaria is 

 that general for Angiosperms. The bundles, however, vary greatly 

 in size and composition in different types, and this is closely related 

 to the needs of the plants in question. A marked variant is seen in 

 aquatic plants, where the need of water-supply is not pressing. There 

 the woody tissue is reduced sometimes to the vanishing point (Elodea) ; 

 in less extreme cases the stele is contracted, and the individuality 

 of the vascular strands is not maintained (Hippnris). In plants with 

 a climbing habit, on the other hand, the vascular strands are isolated, 

 with bands of soft parenchyma between them, while the wood- 

 vessels are large, and the phloem plentiful. A case in point is seen 

 in the Cucumber (Fig. 25). Here, though the vessels are few, their 

 radial rows can be traced, with the protoxylem directed centrally. 

 A few vessels of enormous size associated with pitted tracheides 

 replace the more numerous vessels of herbaceous stems. The most 

 marked modification is in the phloem, which is much more plentiful. 

 A large mass of it with numerous large sieve-tubes is seen in the 



