50 BOTANY 



As a rule, every living vegetable cell has a nucleus. 



Dead cells lose their living protoplasmic contents, and, strictly 

 speaking, should no longer be termed cells, although the name wasfirst 

 applied to them when in that condition. In reality they represent 

 only cell cavities. With their death, however, cells do not lose their 

 importance to a plant. Without such cell cavities a plant could not 

 exist, as they perform for it the office of water-carriers, while at the 

 same time exercising other functions. The necessary rigidity of a 

 plant is also dependent, to a great extent, on the mechanical support 

 afforded by a framework composed of dead cells. Thus the heart of 

 a tree consists exclusively of the walls of dead cells. 



The Protoplasm. — We naturally begin with that substance which 

 constitutes the living plant body, the Protoplasm, also more shortly 

 designated the Plasma. In order to facilitate an insight into the 

 real character of protoplasm, attention will first be directed to the 

 Slime Fungi or fungus animals [Myxomyeetes), a group of organisms 

 which stand on the border between the animal and vegetable king- 

 doms. These Myxomyeetes are characterised at one stage of their 

 development by the formation of a Plasmodium, a large naked mass 

 of protoplasm. 



The plasmodium is formed from the protoplasm of the spores. 

 These spores are unicellular bodies (Fig. 52, a, b), filled with cytoplasm, 

 in which lies a central nucleus, and are surrounded by tenacious cell 

 walls. The spores germinate in water, their contents, breaking through 

 the spore walls, come out (c, d) and round themselves off. A change 

 of form soon takes place ; the protoplasmic mass elongates and assumes 

 somewhat the shape of a pear, with the forward end prolonged into a 

 fine whip-like process or flagellum (e, f, g). Thus the contents of the 

 spore have become transformed into a SWARM-SPORE, which now swims 

 away by means of whip-like movements of its flagellum. 



In addition to the nucleus, which is visible in the front end of 

 every swarm-spore, a vesicle may be seen at the other end, which, after 

 gradually increasing in size, suddenly vanishes, only to swell again 

 into view. This vesicle is a contractile vacuole. The presence 

 of such a contractile vacuole in an organism was formerly considered 

 a certain indication of its animal nature. Now, however, contractile 

 vacuoles have been observed in the swarm-spores of many green Algae, 

 of whose vegetable nature there can be no doubt. 



The swarm-spores of the Myxomyeetes soon lose this characteristic 

 swarm -movement, draw in their flagella, and pass into the amoeba 

 stage of their development, in which, like animal amoebae, they assume 

 irregular, constantly changing shapes, and are capable of performing 

 only amoeboid creeping movements. In the case of Chondrioderma 

 difforme, a Myxomycete of frequent occurrence in rotting parts 

 of plants (Fig. 52), a number of the amoebae eventually collect 

 together (/) and coalesce. In this way, as is also the case with 



