238 PROGRESS OF MICROSCOPICAL SCIENCE. 



in the woody zone. A very thick bark surrouufls the woody zone. 

 Immediately in contact with the latter it consists of a thin layer of 

 delicate vertically elongated cellular tissue, in which the mui'al tissues 

 of the outer extremities of the medullary rays become merged. Ex- 

 ternally to this structure is a thick parenchyma, which quickly assumes 

 a more or less prosenchymatous form and becomes arranged in thin 

 radiating laminne as it extends outwards. The eindermal layer con- 

 sists of cellular parenchyma with vertically elongated cells at its 

 inner surface, which feebly represents the bast-layer of the other 

 forms of Lepidodendroid plants. The rootlets consist of an outer 

 layer of parenchyma, derived from the epidermal parenchyma. Within 

 this is a cylindrical space, the tissue of which has always disappeared. 

 In the centre is a bundle of vessels surrounded by a cylinder of very 

 delicate cellular tissue, prolonged either from one of the medullary 

 rays or from the dcKcate innermost layer of the bark, because it 

 always accompanies the vessels in their progress outwards through 

 the middle and outer barks. It is, he says, evident that all these 

 Lepidodendroid and Sigillarian plants must be included in one common 

 family, and that the separation of the latter fi'om the former as a group 

 of Gymnosperms, as suggested by M. Brongniart, must be abandoned. 

 The remarkable development of exogenous woody structures in most 

 members of the entire family indicates the necessity of ceasing to apply 

 either to them or to their living representatives the term Acrogenous. 

 Hence the author proposes a division of the vascular Cryptogams into 

 an exogenous group, containing Lycopocliacece, Equisetacece, and the 

 fossil Calamitacece, and an endogenous group, containing the ferns ; 

 the former uniting the Cryjitogams with the Exogens through the 

 Cycadece and other Gymnosperms, and the latter linking them with 

 the Endogens through the Palmacece. 



Passage of Corpuscles through the Blood-vessels. — A long and 

 important paper on this subject has been lately read before the Royal 

 Society, by Dr. E. Norris, Professor of Physiology in Queen's College, 

 Birmingham. He says that on a careful consideration of the hypo- 

 theses which have been propounded by Waller, Cohnheim, Strieker, 

 Bastian, and Caton, to account for the curious phenomena in question, 

 it will be found that all these hypotheses fall short in one important 

 particular, inasmuch as they afford no explanation whatever of by far 

 the most singular part of the process, viz. the fact that the apertures 

 through which the corpuscles pass again close up and become invisible. 

 The question, indeed, is not so much how the corpuscles get out, as 

 how they get out without leaving any permanent trace of the aj>ertures 

 through which they have so recently jiassed, and which were so pal- 

 pable during the period of transit. Before proceeding to elaborate 

 his own views, he restates succinctly the various points upon wliich 

 observers are agreed. 1st. Both white and red corj)uscles pass out of 

 the vessels through apertures which can neither be seen before their 

 ingress into or egress from the vessel wall, but only during the period 

 of transit. 2nd. An essential and primary step in the process is, that 

 the corpuscles shall adhere or, more properly, cohere to the wall of 

 the vessel. 3rd. These cohering corpuscles shall subsequently be 

 subjected to pressure from within. 



