THE CELL-WALL. 



^3 



of which may again exhibit numerous layers and the striation already described. In the 

 case of exposed cells which require protection (as pollen-grains or spores), or of those 

 which themselves serve as a protection to other tissues (as cork), an outermost shell 

 (of greater or less thickness) of each cell-wall is transformed into cork or cuticle. 

 When the cells are destined to form a firm frame-work (as in wood-cells), the outer 

 masses of layers become lignified ; in other cases, on the other hand, the outer layers, 

 rarely the inner ones, are transformed into mucilage. Usually an inner layer of the cell-wall 

 remains unchanged in all three cases, and gives the above-mentioned cellulose 

 reactions, while the suberised and 

 lignified shells of the cell-wall 

 may, after previous treatment 

 with alkalies or with nitric acid, 

 also exhibit these reactions; the 

 layers which are transformed into 

 mucilage are the most refractory. 

 Some of the morphological 

 relations here treated of find 

 their explanation only when we 

 study the formation of tissues; 

 but I cannot here discuss the 

 chemical behaviour of the cell- 

 wall ; the reactions must properly 

 be regarded not as chemical tests, 



>ut only as the means of recog- 

 nising a morphological differen- 



iation. The description of some 



Lamples will be sufficient to guide 



le beginner. 

 The pollen of Thunbergia alata 



\. 36) shows that the different 



jvelopment of two shells of a 



ill-wall may go so far that the 

 juticularised shell, the Extine, be- 

 comes actually separated from 

 the non-cuticularised shell, the 

 Intine, which still possesses the 

 power of growth ; by this means 

 it becomes broken up in most 

 cases by fissures previously formed 

 into one or two spiral bands. 

 This can be artificially induced 

 by laying these pollen-grains in 

 concentrated sulphuric acid or potash solution ; the extine then assumes a very beautiful 

 red colour, while the intine in the first case dissolves, in the second case swells a 

 little and remains colourless. In the germination also of many spores {e.g. Spirogyra, 

 Mosses, &c.) the cuticularised exospore becomes completely separated and stripped 

 from the endospore, which still continues to develop; both shells, however — cor- 

 responding to the extine and intine of the pollen-grain — consist, in their actual 

 development, of systems of layers of a single cell-wall possessing different chemico- 

 physical properties. 



FIG. 35. — A pollen-grain of Cucttrbita Pepo, which has emitted a pollen- 

 tube (sf) into a papilla of the stigma. The cell-wall of the pollen-grain consists 

 of a cuticularised extine (<■), and an intine capable of growth (?) ; the latter is 

 greatly thickened at certain places {B i) ; on each thickening -mass the extine 

 forms a roundish lid (d) ; when the pollen-grain is commencing to emit its 

 pollen-tubes, the thick parts of the intine swell, and thus tilt up and lift off the 

 lid-like piece of the extine j one or two of these thickening-masses form pollen- 

 tubes (X 550). 



as in the striae; another term must be employed for the structures now under consideration, and 

 the expression * shells' (Schalen) appears to answer the purpose. 



D 



