94 



MORPHOLOGY OF TISSUES. 



a mass of tissue of characteristic form one or more layers in thickness, the cells 

 of which are distinguished by containing a turbid fluid with drops of oil and resin 

 floating in it, and by their walls being not lignified and usually thin. If the tissue 

 which surrounds the secretion-canal does not increase in size, the formation of the 

 canal or intercellular passage is sometimes altogether suppressed; the secreting cells 

 retain their secreticm, and form gland-like groups of cells instead (Fig. 78, A, D). 



Secretion-canals resemble laticiferous vessels in this respect, that they follow, as 

 a rule, the direction of growth of the organ, even when they do not lie in the fibro- 

 vascular bundles, and also that they penetrate into all parts of the plant, although 

 particular organs, especially the fruit and flower, are destitute of them. When 

 any part of the plant, especially the stem, which is rich in them, is wounded, and the 



Fig. 78.— Transverse section of resin-passagfes (g) at the base of a first year's branch oi Finns sylvesiris {x 5So). A^ B, C 

 passages lying in the periphery of the pith (j^ spiral vessels of a fibro-vascular bundle) ; at A the formation of a passage has not 

 taken place, but the cells destined for its formation are there, their walls h.iving become softer; D wood-cells (h) enclosing 

 a group of resin-cells, not forming a passage (st a medullary ray) ; E part of the wood containing a resin-passage (^) ; next it 

 wood-cells containing starch (am), forming a zone passing from one passage to another. 



canal therefore broken, the contents, forced by the pressure which masses of tissue 

 exercise on one another, escape, in the same manner as latex, collect round the 

 wound, and finally become hard. In this manner are obtained the various kinds of 

 resin, as that of Coniferae, mastic, sandarach, &c., in the same way as dried latex such 

 as euphorbium, lactucarium, opium, &c. 



Sect. 15. The Epidermal Tissue. — A differentiation into epidermal tissue 

 and inner fundamental tissue can evidently only arise in those plants and parts 

 of plants which consist of masses of tissue. In general the contrast of the two 



