28 



THE GARDENER'S ASSISTANT. 



most important part in the growth and develop- 

 ment of new cells from the old, but which for our 

 present purpose we may pass over with the mere 

 mention. One such cell as we have mentioned 

 may constitute the entire plant; the plant and 

 the cell are one and the same, so that in that 



case all the business of life is carried on in that 

 one cell. 



Usually, however, plants consist not of one, but 

 of a large number of cells in association (fig. 1 2). 

 Just as a house is made up of bricks, so a plant 

 is made up of cells. There is, however, this 



Fig. 15.— Melon— Spiral Vessels. 



Fig. 16.— Annular Vessels. Fig. 17.— Spiral and Annular Vessels. 

 Fig. lit.— Dandelion— Laticiferous Tissue. 



Fig. 18.— Celandine— Laticif ?rous Tissue. 



important difference: the bricks are dead things, 

 but the cells, at least in the young state, are 

 full of life. They feed, they grow, they breathe, 

 they secrete, they manufacture substances re- 

 quisite for the nourishment of the plant at a 

 future time, they store up those substances till 

 they are required, they increase in size, and they 

 multiply. 



Some plants, like the simplest Alga? and 

 Fungi, consist of cells, and cells only, that is 

 to say, of cells that are little if at all modified 

 in course of growth. Generally speaking, how- 

 ever, as the work of the plant becomes greater 

 and more complex, and as the outside conditions 

 vary, so the nature of the cell alters also. 



The most common change is a thickening of 

 the cell-wall by deposit on its inner surface 

 (fig. 11). The object of this is to confer strength 

 and power of resistance 

 protect soft tissues in the 

 The thickening layers may be very thin, or they 

 may be so thick as nearly to fill up the original 

 cavity of the cell, as shown in the accompany- 

 ing illustrations. 



In form, also, the cell varies greatly; origin- 

 ally globular, it becomes compressed by its 

 neighbours, and so assumes a polygonal form 

 (fig. 12). Almost invariably some of the cells 

 lengthen into tubes (fig. 20), these tubes being 

 often irregularly thickened, so that thin inter- 

 spaces exist between the thickened portions, 



, and, sometimes, to 

 vicinity from injury. 



whilst not unfrequently the deposit takes the 

 form of a spirally-twisted thread or threads 

 (figs. 15, 17). The tubes just mentioned are of 

 two distinct characters. In the one case they 

 consist of single cells lengthened out, when they 

 are called, if thickened, wood-fibres or tracheitis 

 (figs. 20-22); in 

 the other case 

 the tubes are of 

 composite origin, 

 resulting 



from 



Fig. 20. 

 Tracheids. 



Fig. 21. 

 Fibres. 



Fig. 22. 

 Pitted Tracheids. 



the placing of cells in one line, end to end, and 

 the breaking down of the partitions between 

 them, so that a continuous tube or vessel is 

 formed (figs. 15-19). 



