404 REVIEWS. 



of the apposition of layers at these spots. The bottom of 

 the pit is bounded by the original cell-wall, over which a 

 thin layer is continued ; the sides of the pit are covered by a 

 limiting membrane, which, however, is no continuation of 

 those between the layers of the wall, but is due to an 

 alteration at the edges of these layers as they suddenly stop 

 at the pit. 



We have here, therefore, an example of growth by apposi- 

 tion in its simplest form ; the layers — each of which must 

 be regarded as formed of several lamellae — marking periods 

 of active growth, the separating surfaces indicating pauses 

 in the process. 



In the young embryo sac of Ornithogalum^ the secondary 

 thickening layers arise when the sac is full of tissue, pro- 

 ceeding centripetally from the outer cells ; all stages can 

 therefore be obtained in one section. 



The primary walls of unthickened cells are seen to be 

 clothed by a very delicate layer of protoplasma, bearing 

 " microsomes " which are coloured brown by iodine ; at this 

 period the starch granules begin to disappear, and by the 

 end of the thickening process none remains. It is possible 

 to cause the protoplasm to withdraw, leaving the " micro- 

 somes ^' arranged in series on the cell-wall. Such a sheet 

 of microsomes, embedded in protoplasma, is to be regarded 

 as an incipient thickening lamella. The numerous other 

 details agree essentially with what has been stated, and 

 we may pass shortly over the cases of the pith of Taxo- 

 dium, the endosperms of Phoenix and Stryclmos, the 

 hardened endocarps of many stone fruits, »&c. Many 

 important observations are to be found in this section, as 

 well as in that embracing the investigations of wood- and 

 bast-cells ; and Strasburger finds more and more support 

 for his views here, adducing at the same time difficulties 

 which the theory of intussusception does not at any rate 

 obviously explain. 



Among others the very fine canals or pores passing com- 

 pletely through the walls from cell to cell of the endosperm 

 of Stryclmos should be especially noted ; similar continuous 

 canals occur in other cases, and Strasburger seems inclined 

 to lay some stress on these as of more general occurrence. 

 The remarks on Gloeocapsa are also important. Space 

 does not admit of our dwelling further on this portion of 

 the book, however, and one more example must suffice as 

 indicating the mode of treatment here pursued. 



Professor Strasburger has made an excellent study of the 

 wood-cells in Pinus, and since these have also been investi- 



