54 



MISUTE STIWCTURE. 



fers. Fig. 2M is a diagram showing the structure of 

 these peculiar markings. 



When cells stand end to end, and thin spots are left in the 

 cross-partitions between them, slcvp.-cells ai'O formed. 

 Here, again, the tliin spots finally disappear, thus prac- 

 tically Igniting adjacent cells. Fig. 2G7 illustrates these 

 cells. Here J9S represents the shrunken protoplasm (lifted 

 off the perforated cross-pai-tition at sii) ; si, a sieve-plate 

 on tlie side-wall. On the right is a view of tlie sieve- 

 like wall. 



It sometimes happens that the thickening takes place through- 

 out tlie length of a cell but in its angles only. Cells of 

 this kind, wliicii are often found immediately under the 

 surface of the stem in the higher plants, are called collen- 

 chrjma cells. Fig. 268 is a transverse section of a petiole 

 of a Begonia leaf, showing collenchyma cells ; e is the 

 epidermis, chl chlorophyll granules. 



Besides the markings on the inside, cells often show mark- 

 ings on the outside. The pollen-grains of tlie Mallow, 

 for instance, are seen under the microscope to be covered 

 with pointed projections. Other pollen-grains, also, ex- 

 hiljit outside markings of different sorts. 



Tlie tliickening deposit may be so excessive in some cases as 

 to almost completely fill up the cavity of the cell (Fig. 

 '2()n). The shells of nuts and the tough coatings of seeds 

 consist of cells of this kind ; but even in these cases the 

 wall may be seen to be traversed by slender pores or 

 canals, either simple or branched, radiating from the 

 centi-e of the cell. To these hardened cells the name 

 scl(:r(in<-]iy)iia is applied. 



The Contents of Cells. 



If j'ou look at Fig. 252, or, better still, if you have the oppor- 

 titnity of viewing a Moss-leaf through a good microscope, 

 3'ou will see that in the protoplasmic lining of the cells 

 there are numerous greenish, rounded granules. These 

 are the bodies to which the green parts of plants owe 

 tiieir colour. They ai'e called chloroi^hyll-yi^anulcs, and 

 consist of protoplasmic matter in which particles of green 

 colouring matter are embedded. The colouring matter 

 itself is chlorophj-ll, and may be dissolved out of the 

 granules, leaving the latter as ordinary i^rotoplasin. 

 Almost without exception chloropliyll requires the action 

 of sunlight for its production, and tlie clilorophyll dis- 

 appears from green parts when sunlight is withdrawn, as 

 is well seen in the process of bleaching celery. In many 

 of our brightl}^ coloured foliage-plants the chloroph^'ll is 

 concealed from view by other colouring matters. In 

 flowers various colours are found in the protoplasm, but 



Fig. 2G6. 



Fiff. 267. 



Fig. 2fi8. 



