SCIENCE- GOSSIP. 



245 



found small drops of the intrinsic secretion of 

 the lining layer of cells, which latter are of 

 the nature of a glandular endothelium. In the 

 Gymnosperms {Pinus sylvestris) we find in the 

 cortex of the stem similar structures ; but we 

 may recognise here two definite layers in each 

 canal. (1) An outer layer of thick-walled sclerised 

 elements forming a " strengthening layer " ; and (2) 

 the internal endothelial lining, with cells con- 

 taining very granular protoplasm (see fig. i. 1). The 

 intrinsic endothelial secretion in this case is a 

 true resin, which is probably an oxidation pro- 

 duct of abietic acid. The leaf of Pinus has a 

 series of these resin-canals distributed throughout 

 the mesophyll lying just internal to the hypo- 

 dermis ; but in this case the strengthening layer is 

 usually composed of more than one layer of 

 sclerised elements, and moreover are not by any 

 means so regularly disposed as is the case with 

 those in the cortex of the stem (see fig. i. 2). 



Resin-canals in Section prom Different Parts 

 of Plants. 



(1) From cortex of stem (Pinus). a. Strengthening layer. 

 b. Endothelium. (2) From mesophyll of leaf (Finns'), c. Cells 

 of mesophyll. (3) From cortex of stem (Aethusa cynapium). 



The main structural difference that is to be ob- 

 served between the canals of Pinus and those 

 of the Umbelliferae is the fact that the 

 strengthening layer is so much more developed 

 in the former than in the latter ; in Aethusa 

 it is merely represented by a ring of smaller 

 cortical cells than the rest, the walls being 

 perhaps slightly thickened. The chemical differ- 

 ences in the respective secretions of their en- 

 do thelia are however great, but they need not 

 be discussed here, as they are somewhat complex. 

 One reason for the existence of these canals is 

 obvious — namely, they, by virtue of their secretion, 

 form a very efficient protective system in pouring 

 out resin, etc., over buds, and so preserve delicate 

 structures through the cold months. Also, when- 

 ever a shoot is lopped off, either accidentally or 

 intentionally, the resin, which is under consider- 

 able pressure in the canals, wells up over the cut 

 surface, and so protects it from the outer air, 



laden with spores of various saprophytes, which 

 might find a good substratum on freshly cut 

 tissues. Another explanation is, however, possible, 

 and one that perhaps is founded on rather more 

 biological grounds. During the progress of meta- 

 bolism the plant elaborates certain compounds 

 which must be looked upon in the light of excre- 

 tions. Such a one is abietic acid. These excretions 

 if allowed to remain in the cells might effectually 

 hinder anabolism, and so something must be done 

 to get rid of them if they are produced in any 

 appreciable quantity. A stimulus is thus set up, 

 and the protoplasm, influenced by a sort of 

 " directive action," proceeds to create a system of 

 canals lined by cells whose special function is to 

 utilise the excretions and form a substance (resin) 

 which will be of some use to the plant. This is 

 only another instance of the adaptability of plants 

 to adverse circumstances — one, moreover, fully 

 illustrating the great underlying principle that 

 " action and reaction are equal and opposite." 

 Resin canals of the same structure as those of the 

 cortex are to be found in the first year's wood in 

 Pinus, and in the root are seen lying in the funda- 

 mental tissue between the forks of the Y-shaped 

 protoxylem. In the latter the strengthening layer 

 is not usually composed of such thick-walled 

 elements as is the case in those of the stem. 



Symmetry in Internal Structure. — It is 

 F/c.L 



F'c.& 



Hexagonal Symmetry Illustrated. 



(1) Young cells of pith in bud ; these preserve more or less of 

 their oval shape. (2) Cells of pith in stem. The majority of 

 cells in section present six walls. (3) A perfectly symmetrical 

 hexagonal cell from cortex of same plant ( Umbelliferae). 



apparent to every student of the histology of plant- 

 structures that there are definite laws governing 



