Structural Botany : Growth of the Shoot. II. 



A typical Land Plant, as derived from submarine vegetation, is a more complex 

 organism in that the first problem is how to obtain sufficient Nitrates, Phosphates, 

 and other compounds as ' food-salts ' to carry out proteid-synthesis, when these are 

 only present in the soil. This involves the necessary differentiation of an absorbing 

 root in the ground, with subaerial shoot', the function of the latter being to produce 

 and display as much body-surface as possible to air and sunlight. Since the 

 absorbing cells are in a solution of soil-water (often little better than tap-water), 

 a current has to be maintained from the roots to the leaves (Transpiration-current), 

 and enormous quantities of water require to be evaporated at the surface of the 

 leaves (Transpiration). 



Hence the Primary distinction iii land-plants of Leaves, Stem^and Root', the 

 first being the essential photosynthetic and proteid-synthetic regions of the organism. 

 The Leaf is the Laboratory of the Plant ; with conductive system leading to it, as 

 also away from it, in the form of definite strands of vascular tissue known as 

 Vascular Bundles (V.B.) of Xylem and Phloem. 



The main axis normally grows erect, at right angles to the surface of the ground, 

 to give optimum space-distribution ; bearing leaves as laminate extensions of the 

 soma, and branches repeating similar organization ; thus giving a * tree habit ' of 

 diffused growth spread over considerable space, as opposed to the concentrated and 

 condensed body of a locomotile animal. The Plant-axis also exhibits continuous 

 growth, by definite 'growing-points', which may go on producing new leaves and new 

 axes so long as nutrition is satisfactory ; i.e. the plant continues to grow at the ends. 



Growing Point : essentially a mass of undifferentiated growing and dividing 

 cells (meristem); exhibiting phases of (i) centric growth, i.e. around a point; 

 (2) unilateral distribution, i.e. on one side only; maintaining the growing apex at the 

 end of a longitudinal axis ; (3) retarded, since the stem attains a definite ' adult ' size. 

 Increase is effected by the growth and sub-division of each individual cell in irregular 

 sequence : any cell may be dividing, but the general effect and dome-shaped 

 outline are retained ; i. e. the mass grows as a whole, and all the protoplasts are in 

 living communication, as typical parenchymatous cells, or in this case, embryonic 

 tissue, practically immortal, and including generalized germ-plasm. 



Mechanism of Cell -Division : The nucleus divides first, presenting phenomena 

 of Mitosis : In higher plants and animals the nucleus presents complex organization 

 of plasmic ground-substance, with fluid in vacuolations, and framework of linin, 

 with chromatin distributed in a reticulum of chromosomes. The latter are of constant 

 number for a given species, and display a certain degree of individuality, being 

 capable of independent division by longitudinal fission. The mass of the nucleus is 

 fluid, and is held in spherical form among the cell-cytoplasm by surface-tension, and 

 is covered by a plasmatic film (nuclear membrane), as its ' resting-stage ' in the 

 metabolizing cell. In Mitosis the membrane disappears ; the linin arranges a ' spindle- 

 mechanism ', with ' spindle-fibres ' from two ' poles ', and the chromosomes separate 

 (prophase), arrange in the ' equatorial plate ', and each divides : spindle-fibres attach 

 to the halves, and the ' daughter-chromosomes travel to the poles (anaphase), there 

 uniting to build the new reticulum of the 'daughter nuclei' (telophase). In the 

 equatorial plane, across the central spindle-fibres, a zone of demarcation of the fields 

 of the two new centres is marked by a deposit of polysaccharide waste, as the first 

 indication of the dividing wall : this extends to the older walls, and constitutes the 

 primary septum. Later deposits of cellulose on this wall commonly present cleavage- 

 effects, with the production of intercellular spaces at the angles (more or less) ; the 

 original septum is distinguished as the ' intermediate lamella ', and the intercellular 

 spaces are required for the aeration of the tissues. 



Apical Differentiation. As cell-division proceeds behind the growing point, 

 organization is shown in the layering of the tissues, following periclinal construction- 

 lines, and indications of tissue-systems appear : 



(1) A peripheral layer controlling relations with the outer air, as epidermis. 



(2) A generalized intermediate region (cortex). 



(3) A central conducting and skeletal region, conveniently included as the Stele. 

 The early distinction of these regions close to the growing-point led to the 



