MORPHOLOGY OF HIGHER PLANTS. 197 



may consist of a more or less branching net-work formed by the 

 anastomosing of a number of cells, as in Taraxacum (Fig. loi, 

 D). It is distributed or associated with the sieve in nearly all 

 parts of the plant; in the earlier stages the cells contain proto- 

 plasmic cell-contents which later disappear or are with difficulty 

 distinguished from the other substances already enumerated as 

 present in the milk-juice. 



Secretion Cells or Canals. — In Sanguinaria there occurs a 

 rudimentary laticiferous tissue, most of the juice being contained, 

 however, in special parenchymatous cells, which may be more or 

 less isolated, or arranged in irregular longitudinal rows. Cells 

 of this character are known as secretion cells and usually contain 

 oil, resin, tannin, calcium oxalate, mucilage (Figs. 98; loi, B), 

 etc., instead of substances which form an emulsion or milk-juice; 

 these cells are distributed in all parts of the plant, and include the 

 epidermal cells and glandular hairs. The walls usually consist of 

 cellulose but may have lamellae of cutin and suberin, the latter 

 being found particularly in the oil-secretion cells of rhizomes, 

 roots, barks and fruits (Figs. loi, B; 212 ; 236; 250). 



In some instances mucilage cells containing raphides occur in 

 longitudinal rows resembling the secretion cells of Sanguinaria ; 

 in some of the ferns, the barks of elder and locust, and leaves of 

 the Crassulacese, the tannin-cells are very much elongated, resem- 

 bling the simple laticiferous cells in the Asclepiadaceae. 



Oils, resins, mucilage, gum-resins and allied products occur 

 quite frequently in special reservoirs or cavities formed as already 

 described (p. 178). 



INNER STRUCTURE OF MEMBERS OR ORGANS. 



Point of Vegetation. — Plants are distinguished for the most 

 part by having distinct growing points known as vegetative points. 

 These occur at the apex of shoots and roots and at definite lateral 

 points, being in the stem near the surface and in roots beneath 

 the endodermis. The walls of the cells in these regions are very 

 thin and consist almost entirely of cellulose. The cells are com- 

 pactly arranged and are more or less polygonal or somewhat 

 elongated. They are rich in protoplasm, capable of rapid divi- 



