CONDUCTING SYSTEM. 



Ul 



i: 



^'l 



Fig:. 55 



Sieve tubes from Poppy capsule. 



posed in tho form of spirals. (8,81,83). 

 There may bo from one to several ou the 

 inner wall of a single duct. The spirals 

 generally coil from ri^'ht to left, but iu 

 some cases two or more will coil in oppo- 

 site directions, forming a double set of 

 spirals. Some variation of this form is 

 found in the corollas of most flowers, 

 in the stem of Conium, in Bulbns Scillae, 



etc. 

 Reticulated ducts have net-shnped 



thickenings on the inner walls, (n.) 

 Sometimes they are quite regular, form- 

 ing ladder-like markings, and again they 

 nre extremely irregular. 



Pitted ducts are quite different from 

 the preceding forms in that their walls 

 are wholly thickened but densely per- 



forated with pores, either bordered or 

 simple, or both, and in form round, nviil 

 or Blit-shaped. Typical pitted dxictn mny 

 be seen in Radix Glycyrrbiza, Tu1>*t 

 Jalap. (Fig. 54.) 



Primary vessels are generally annular 

 or spiral, presenting a greater nnthick- 

 onod interior surface, but the VL;Siiel8 of 

 secondary growth are generally reticulat- 

 ed or pitted. 



Pnrencbyma. Numerous parenchy- 

 ma tic ceils may he found intcr.sijcrsed 

 nnuuig tbc t'li^rnents of the Xrlem tbat 

 S4TVC f<»r pni'iHtst's of wat*'r conduction. 

 Aii a rule, they offer ilttb' in the way of 

 diagnostic characteristics. 



2. Lepfom, or the Conductors of Plastic 

 ^fatcrials. The leaves and young grow- 

 ing stems contain plastic uiatcrials 

 which, when combined with the watir 

 and inorganic salts brought by the ducts 

 and trat^heids from the roots, serve the 

 purposes of anabolism. The I.rptom is 

 a series of tissues (according to llalM^r- 

 landt, the conducting elements of tho 

 Phloem), whose special function it is to 

 convey these worked-over ma teria Is 

 down from the leaves, where the active 

 transformations take place, to thoso 

 parts of the plant where they are needed. 



The Leptom consists of Sieve Tubes, 



Cambif orm Cells and • Accompanying 

 Cells, in fact, all the elements of the 

 Phloem, except the bast fibers, which be- 

 long to the protection system. 



Lactiferous Tissues, or Milk Tubes, 

 are in part plasma-conducting vessels and 

 in part secretory organs. 



(a) Sieve Tubes are long cells whose 

 end walls are perforated, forming a 

 sieve-like plate through which the plastic 

 materials cuu pass. These sieve plates 

 may be horizontally arranged if the ends 

 of the cells were originally straight, but 

 thoy are generally much inclined. As n 

 rule, the sieve tubes in primary tissues 

 are more npt to be horizontal, while 

 those in tissues of secondary growth are 



apt to be inclined. 



The walls of sieve tubes are generally 

 thin and composed of celhilose. The 

 sieve tubes vary greatly in size, but as a 

 rule they are smaller than the vessels. 

 They are often very irregular in form, 

 being large and swollen at the points 

 where the sieves occur and much con- 

 tracted between. (Fig. 53,) 

 Their contents are plastic, consisting 



