36o SCIEXCE PROGRESS. 



(Bthiopica, in the peduncle of N^arcissus pseudo-Narcissus, 

 etc. In N^aj'cissus it occurs in the intercelkilar spaces. 



As the wall of the vouno- cell increases in thickness the 

 secondary laminae show a certain difference in composition, 

 pectoses being most abundant in the layers nearest to the 

 middle lamella and cellulose most prominent in the newest 

 layers nearest to the protoplasm of the cell. 



The dissociation of the young tissues shows the interest- 

 ing peculiarity indicated by Payen, the substance joining 

 the cells not beino- the same throuohout. If transverse 

 sections be taken, the dissociation easily takes place in the 

 radial direction but not so readilv in the tang^ential one, so 

 that radial rows of cells are separated from each other. If 

 a longitudinal section of the growing apex of a stem or root 

 be examined the lines of easiest cleavage can be seen to 

 converge towards the initial cells of the apex, each group 

 being wrapped round by a thin membrane. 



We may now turn to the question of the composition of 

 the primitive cell-wall. Is this originally homogeneous? If so, 

 it mavbe rec^arded as beinq-from the first formed of an intimate 

 mixture of cellulose and pectose, forming a kind of cellulo- 

 side approaching a glucoside in nature. On this hypothesis 

 it becomes thickened with layers of the same nature, and 

 the middle lamella is not at first present as a differentiated 

 region. Soon by a kind of intussusception a deposit of 

 calcic pectate is produced in the middle of the wall ; or. such 

 a deposit may arise from a decomposition of the pectose 

 under the influence of the acid sap and the infiltration of 

 soluble calcic salts. 



On the other hand the primitive wall may be from ti'.e 

 first heterogeneous the first lamina being formed of calcic 

 pectate, which becomes very speedily covered on its two 

 faces by layers of the mixture of cellulose and pectose. On 

 this hypothesis the middle lamella is universally present, 

 and in fact constitutes the " primary membrane ' of the 

 writers immediately preceding Naegeli. 



Whichever hypothesis may be adopted there is no doubt 

 that in soft tissues at the moment when cell division ceases 

 the wall can be shown to consist of a middle lamella of 



