PROaRESS OF MICROSCOPICAL SCIENCE. 239 



From this he proceeds to consider the various physical conditions 

 involved in the question, and eventually he states that all that is 

 essential for a rigid or plastic body to pass through a colloid film is : 

 — 1st, an intimate power of cohesion, either mediately or immediately, 

 between the film and the body ; 2nd, a certain amount of pressure from 

 within ; 3rd, power in the substance of the film to cohere to the sur- 

 face of the body (or to some intermediate matter which already coheres 

 to the surface) during its passage ; 4th, cohesive plasticity of the parti- 

 cles of the material of which the film itself is composed, so that the 

 breach in it may again become reimited as it descends upon the oppo- 

 site sxirface of the body which is being extruded. These conditions 

 he thinks are prevalent in the case of the minute blood-vessel and the 

 corpuscle; since he considers that the passage of the corpuscle 

 through the vessel is precisely analogous to the passage of a body 

 through a soap bubble. Unfortunately in parts the author's language 

 is not sufficiently clear to enable one to find his exact meaning, but we 

 believe that we have found it correctly as above. 



The Structure of Lepidodendron is stated by Professor Williamson 

 to be in the case of Lepidodendron selaginoides as follows : — It consists 

 of a central medullary axis composed of a combination of transversely 

 barred vessels with similarly barred cells ; the vessels are arranged 

 without any special linear order. This tissue is closely surrounded 

 by a second and narrow ring, also of barred vessels, but of smaller 

 size, and arranged in vertical lamin?e which radiate from within out- 

 wards. These laminae are separated by short vertical piles of cells, 

 believed to be medullary rays. In the transverse section the inter- 

 sected mouths of the vessels form radiating lines, and the whole struc- 

 ture is regarded as an early type of an exogenous cylinder ; it is from 

 this cylinder alone that the vascular bundles going to the leaves are 

 given off. This woody zone is surrounded by a very thick cortical 

 layer, which is parenchymatous at its inner part, the cells being with- 

 out definite order ; but externally they become iirosenchymatous, and 

 are arranged in radiating lines, which latter tendency is observed to 

 manifest itself whenever the bark-cells assume the prosenchymatoiis 

 type. Outside the bark is an epidermal layer, separated from the rest 

 of the bark by a thin bast-layer of prosenchyma, the cells of which 

 are developed into a tubular and almost vascular form ; but the vessels 

 are never barred, being essentially of the fibrous type. Externally 

 to this bast-layer is a more superficial epiderm of parenchyma, suj)- 

 porting the bases of leaves, which consist of similar parenchymatous 

 tissue. Tangential sections of these outer cortical tissues show that 

 the so-called " decorticated " specimens of Lejndodendra and of other 

 allied plants are merely examples that have lost their epidermal layer 

 or had it converted into coal, this layer, strengthened by the bast tissue 

 of its inner surface, having remained as a hollow cylinder when all the 

 more internal structures had been destroyed or removed. — Proceedings 

 of Boyal Society, vol. six., No. 129. 



VOL. VI. 



