412 REVIEWS. 



cellulose results from the breaking down of protoplasm as a 

 split product, the elements not necessarily being identical 

 in all cases, however, as seems indicated by the differences 

 in character of the membranes produced. Whatever 

 opinions may be held as to the value of these considerations, 

 it is clear that Professor Strasburger has made an excellent 

 attempt to bring a large series of hitherto independent facts 

 into relation with one another and with new discoveries ; 

 that the exact nature of the " microsomes " themselves is 

 in the highest degree problematical, it is not necessary to 

 remark. Minute granules in a protoplasmic matrix is 

 always a vague phrase, and we have seen how much Stras- 

 burger's interpretation of the nature of the " microsomes " 

 has changed ; nevertheless, here are other remarkable rela- 

 tions of time and position, and the large number of facts 

 explained is here, as elsewhere, to be kept in mind as a test 

 of the worth of the hypothesis. Further investigations may 

 be anxiously watched in these directions. 



In the section treating of " das Fldcheyiwachsthum der 

 Zellhaiite und die Fcdtenbildung " we meet with further 

 evidence of Professor Strasburger's indefatigable powers of 

 observing and thinking, two operations not always found 

 together. This part of the book is for us of especial interest. 

 It is well known that the growth in surface, &c., of mem- 

 branes has always offered peculiar difficulties, which were 

 considered hardly explicable by the " apposition " hypo- 

 thesis, and therefore of especial weight as evidence for the 

 theory of growth by " intussusception.'^ Strasburger sets 

 out by stating that the results concerning the thickening 

 processes render it in the highest degree improbable that a 

 different mode of growth is followed here, claiming an a 

 priori argument for apposition ; and, indeed, all must allow 

 that if the microsome-bearing protoplasm sheets pass 

 directly into lamella?, it is not easy to suppose that soluble 

 carbohydrates, &c., pass into a layer and crystallise in 

 order to allow of its increase in surface. 



Schmitz first pointed out the possibility that by the 

 stretching and pressing together of layers by the growth of 

 daughter-cells, the walls of mother-cells may become 

 extended. 



In Gloeocapsa this occurs until the outer layers are 

 thrown off; in Cladophora the pressure and extension pro- 

 duce a homogeneous sheath. In such cases the continual 

 stretching is accompanied by apposition of new strengthen- 

 ing lamellae, and it becomes a question of how far the outer 

 layers can endure the process ; very often they become 



