PB ACTIO AL GUIDE TO GABDEN PLANTS 



Other plants, however, which may hve for very many years, like some 

 of the American Aloes, also fruit or produce seeds only once in a life- 

 time and then die. These are also said to be monocarpic. Nearly all 

 our hardy herbaceous perennials, and woody perennials like trees and 

 shrubs — Apples, Pears, Plums, Oaks, Ashes, Beeches &c. — fniit or 

 produce seeds year after year for several generations, and are therefore 

 said to be polycarpic. 



It little matters, however, whether plants be annuals, biennials, or 

 perennials, whether they produce only one crop of seed or many, they 

 are all governed by the same laws of Growth. 



Plant-cells. — To give the reader a better idea of how this process 

 called growth takes place, it is necessary to point out that plants are 

 made up of cells and tissues. Some plants, indeed, like the green 

 Protococcus seen on damp walls, the mould on old leather &c., are very 

 simple in their structure and often consist of one cell only. And it is in 

 the contents of the individual cells, the presence of which was discovered 

 in 1667 by Robert Hooke, that we must look for the origin of growth. 



Protoplasm. — Every plant cell in a young stage is filled with a 

 slimy jelly-like substance to which the name of Protoplasm was given 

 in 1846 by a German botanist, Hugo von Mohl. This protoplasm is 

 practically the seat of life. It is constantly undergoing more or less 

 rapid changes in composition, absorbing new food, digesting it, and 

 expelling all waste or worn-out materials. When the cells are young 

 they are completely filled with it, but as they grow old the protoplasm 

 begins to break up into strands, leaving spaces in between which 

 become filled with watery sap absorbed and drafted up by the roots. 

 By-and-bye the protoplasm recedes from the centre of the cell to line 

 the cell walls, and ultimately vanishes altogether with age, the refuse 

 from it going to make the cell walls thicker and harder, and producing 

 what we know as wood or fibre. 



Before this stage is reached, however, the protoplasm, or rather 

 the central portion of it called the nucleus, divides and forms a new 

 cell. This, like its parent, becomes surrounded with a cell wall, and 

 then becomes practically an independent individual working out its 

 own life history in the same way. When it is remembered that plants 

 are made up of millions of these cells containing protoplasm, and that 

 with the advance of age each mass of protoplasm is capable of pro- 

 pagating itself by division, it is not so very difficult to account for what 

 we understand as growth— how plants often attain enormous heights, 

 and the trunks of trees great diameters — according to their nature. 



Although each mass of protoplasm is as it were completely enclosed 

 within its own cell walls, it is not, however, cut off from all communi- 



