SCIENCE- GOSSIP. 



329 



MECHANICS OF CONDUCTION OF SAP 



By Hakold A. Haig. 



{Concluded from page 306.) 



P ET us now inquire more closely into the nature 

 -*— ' of both cell-wall and parietal protoplasm ; 

 the former of these is, in the root-hairs, composed 

 entirely of cellulose, the molecules of which are 

 of a considerable size. It has been supposed that 

 between certain groups of these molecules there 

 exist spaces, the so-called " micellae " which are 



Fig. 1. A root-hair, showing : — a, cell-wall ; b, protoplasm : 

 c, nucleus ; <l, vacuoles ; e, original cell from which hair arose. 



of a size comparable with the molecular group. 

 Between the constituent molecules of each group 

 still smaller spaces are supposed to exist, these 

 being the "tagmata" of certain botanists. It is 

 quite reasonable to suppose that these spaces do 

 exist ; at any rate, the micellae are probably 

 present, and the supposition affords a ready ex- 

 planation of the absorption of salts and water; 

 but some salts that are positively injurious to such 

 a cell-wall or to the protoplasm, or substances 

 which have a very large molecule (colloid), do not 

 appear to have the power of getting through a 

 membrane of cellulose. It is, in fact, only salts 

 with a molecule comparable in size with the 

 micellar spaces or the tagmata that can get 

 through, and so be submitted to the discriminative 

 power of the primordial utricle. 



As far as the intimate structure of this latter 

 layer goes, but little is known, except that it is 

 made up of two parts — one, the "ectoplasm," being 

 that nearest the cell-wall, and which probably 

 exercises the power of selection ; the other, the 

 '• endoplasm," that is connected with the nutrition 



of the cell. Whether, however, spaces comparable 

 with those in the cell-wall really exist, or whether 

 a real affinity is exerted in the case of the ecto- 

 plasm, is not known. It is conjectured, however, 

 that a sort of sifting process also goes on here, but 

 there has not been much recent research upon the 

 subject. The actual process will not be elucidated 

 except by the aid of extremely refined experiments. 

 It is probably due to that "vital activity " which is 

 supposed to exist in all plants. 



We must now answer the question, What hap- 

 pens after this raw sap has been taken into the 

 root-hairs and other epiblemal cells in their 





Fir;. 2. Trans-section across young stem (cucurbita), to show 

 arrangement of the vascular elements : — ep., epidermis ; col., 

 collenchyma ; cor., cortex (these three constitute the " bark ) " : 

 &., bast-fibres; end., endodermis ; ph., phloem: .r. cambium ; 

 .111., xylem (reticulate and pitted vessels); p.rr., protoxylem 

 (annular and spiral tracheides). 



vicinity.' The problem will be found to resolve 

 itself into the task of determining the nature and 

 structure of the other elements occurring both in 

 root, stem, and leaves. Those elements constitute 

 what is known as the "vascular system "of the 

 plant, and are contained in the fibro-vascular bundles 

 which in Dicotyledons (') are composed of the two 



(1) It does not matter whether we take Monocotyledons or 

 Dicotyledons ; the essential elements are of course present in 

 both. 



M 3 



