174 PRACTICAL BOTANY 



them moist, and sheltered from direct sunlight ; they will then 

 germinate, and after a few weeks the surface of the soil will be 

 found to be covered with small, green, flattened bodies, each of 

 which is an individual prothallus. 



From time to time, pick off some of the young prothalli with 

 a needle from the surface of the soil : by this means a series of 

 observations may be made which will illustrate successive 

 stages of development of the prothallus. 



II. Examine a single, fully-grown prothallus with the naked 

 eye, and observe 



1. The form, which is flattened, and more or less kidney- 

 shaped, with a depression of the margin, at the base of which 

 is the organic apex of the prothallus. Note that the central 

 part of the prothallus is often perceptibly thicker than the 

 periphery : this thicker part is called the cushion. 



2. The position of the prothallus while growing : it is usually 

 oblique to the surface of the soil. 



3. The root-hairs, which spring from the under surface of 

 the cushion, and run downwards into the soil. 



4. The green-colour, due to the presence of chlorophyll : 

 the prothallus is thus capable, under suitable circumstances, of 

 carrying on the process of elaboration of fresh organic sub- 

 stances. 



III. Wash a fresh, well-developed prothallus carefully in 

 water, so as to remove the soil from the root-hairs : mount it 

 whole in water, with the lower surface directed upwards, and 

 examine it with a low power. Observe again the chief points 

 seen above with the naked eye, which are now more plainly 

 visible, and note especially 



1. The form and structure of the cells in the lateral, thinner 

 portions of the prothallus ; they are polygonal, and have thin 

 cellulose walls, and protoplasm containing a nucleus and 

 numerous chlorophyll-corpuscles : the cells at the margin are 

 often extended as hair-like outgrowths. 



2. The cells composing the cushion are of similar structure, 

 but are aggregated in a mass more than one layer of cells in 

 thickness : many of the cells will be seen to have grown out as 

 root-hairs-." 



