30 BOTANY OF THE LIVING PLANT 



such as oil-globules, starch-grains, or crystals in the cytoplasm. 

 But the most marked change is in the disappearance of the protoplasmic 

 body itself, so that it is not represented in the mature structure. 

 This is found in the vessels of the wood, and in ^ome other tissues 

 (Fig. iS, B). Since the vital activities reside in the protoplasm, those 

 tissues where it is absent, consisting only of cell-wall, are no longer 

 actively living, though passively they may still perform functions 

 important in the life of the plant. 



(6) Changes in the Plastids. 



These bodies are minute, and difficult to see in the young cells 

 (Fig. 9). But as the cells mature they may become more numerous 

 by division, and more prominent by their size and colour. In many 

 cells of vegetative parts they turn vivid green, and are called chloro- 

 plasts, or chlorophyll- corpuscles (Fig. 72). They are present in myriads 

 in any green leaf, and collectively give the green colour to the parts 

 in which they occur. Other plastids may take red, or yellow colours, 

 as in petals, or in fruits, and they are called chromoplasts. Others 

 remain colourless, and are called leucoplasts, or starch-forming cor- 

 puscles (Figs. 79, 81). 



By such changes as those described the young embryonic cells 

 may be transformed into the various tissues that make up the mature 

 parts. Originally these were all alike ; as they become mature they 

 are liable to be differentiated and specialised for different functions. 

 It may be held as probable that what is seen in the individual develop- 

 ment is a reasonable guide to what actually took place in the evolution 

 of the race. It is probable that plants with little or no differentiation 

 of tissues, that is Cellular Plants such as the Algae, preceded in the 

 history of Evolution the more complex Vascular Plants, such as the 

 ordinary Land Plants structurally are. 



Continuity of Protoplasm. 



The cells of living tissues all share in a common physiological life, 

 and are in intimate relation to one another. The cell -walls separating 

 adjoining cells do not form complete barriers between the protoplasts. 

 In most mature tissues a Continuity of Protoplasm may be demon- 

 strated, from cell to cell. It is established by means of fine connecting 

 threads. These traverse for the most part those thinner areas of 

 pit-membrane, where the distance to be traversed is the shortest- 

 Occasionally they may also extend through the thicker regions of the 



